Been messing around with a problem for the last few days.
There's a gray area between fully locked and fully unlocked. In that region, although the slide is moved back slightly (a few mm) the locking surfaces are still partially engaged.
As long as the slide and barrel are locked together at the moment of the firing impulse (so that conservation of momentum sees the total weight of the slide & barrel instead of just the weight of the slide) the slide velocity will be essentially unchanged compared to full lockup. The barrel will unlock a tiny bit early because the slide is already back a little, but not really a long time quicker because even though the slide has a head start, the slide velocity should be normal.
At what point does the amount of locking surface engaged become so small as to be unsafe?
80%? 50%? 10%?
As you said (I think), if the slide is not fully forward when the shot is fired then the action will unlock early. I can't speculate on what percentage of engagement is unsafe, but I can say that if the action unlocks before the bullet leaves the barrel and the pressure drops you will have an evil high- pressure event in the ejection port.
When I read your post I took each of the five pistols within arm's reach and tested to see how far back the slide would go before the action begins to unlock. It seemed to me like a pretty fair distance (precision measurement term), long enough that the slide was well beyond the trigger disconnect point. Based upon that I surmise that manufactures don't consider any initial disengagement to be safe.
John, unless there's a linkdown/drop timing issue, the barrel won't drop until it's been drawn to the linkdown point... which in most tilt-barrel designs, that's about 1/10th to 1/8th inch. By that point, the bullet is gone and there's no more need to maintain the lock between barrel and slide.
I'll go get a link that you can see just how short the slide travel is when bullet exit comes. Several interesting clips on it... but the last stop-action sequence just before the revolver shows it more clearly than the others. Very interesting is the AR15's "Bouncing Betty" bolt engagement, which pretty much supports the theory that true "lock" doesn't occur until the pressure hits.
Most interesting is the way that the rounds bounce around in the magazine of that close-up 1911 firing sequence...
Generally speaking... most smiths are good to go with 80% vertical lug engagement... thought I prefer 90% as a minimum... assuming that the first lug and one of the others is engaged equally in the horizontal plane.
Stand by...
http://www.trippresearch.com/media/movement/hispeedgateway.html
On the design I've been working with, the barrel has already dropped by a measureable amount by the time the barrel has been moved back by the 0.1" to 0.125".
Is the golden rule: "Bullet out of the barrel--unlocking is safe."? That's been my understanding.
What I'm looking at is a pathological case. "If something sets off the round with the barrel partially dropped, at what point is the locking engagement insufficient for safety?"
I think that perhaps that's only part of the question. The other part is: "If the slide/barrel are already 0.1" to 0.125" to the rear when the round is set off, will the unlocking happen before the bullet exits?"
What's the general rule for determining the average velocity of the bullet in the barrel? Is there a rule of thumb based on the barrel length and muzzle velocity?
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On the design I've been working with, the barrel has already dropped by a
measureable amount by the time the barrel has been moved back by the 0.1" to
0.125".
Hmmmm
Dropped doesn't mean unlocked. As long as the brechblock and barrel don't separate... it's still locked. As for the surface area, as the bullet progresses, pressures fall off rapidly because of the increasing volume of the cylinder... so it may be a matter of only about 10% of the peak pressure... which was generated much earlier. So... theoretically... if your lug engagement is 10% of its total at that point, and the pressures are at 10% of the peak values... the lockup should be equal to 100% vertical engagement at peak pressure.
On the remaining velocity question... You may want to read the thread I started on the velocity experiment with two identical revolvers. I'll see if I can find it.
As a rule... with the autopistol calibers, you can figure on about 30-35 fps per inch of barrel gained or lost.
Using a 5-inch .45 ACP barrel as a yardstick, you've got 4.1 inch of rifling. If the MV is 850 fps, and the rule of thumb is accurate... you'll get at most about 140 fps from the rifled portion. That leaves 710 fps unaccounted for. If we adjust, and assume that most of the velocity comes within the first half- inch of bullet travel... we're down to 3.6 inches of barrel to gain the remaining velocity. That works out to be about 35fps X3.6 = 126 fps. Now we've got 724 fps to account for... which is about right.
So... Theoretically... Peak pressure and the first 724 fps comes within the first half-inch of bullet travel... and whatever recoil impulse is generated from that aceleration is also generated within the first half-inch of travel, and the greatest shearing force on the lugs also occurs at this point. After that, things start to diminish quickly.
Here's the link to that thread: thread
In the Gun Digest Book of the 1911 vol. 2, Patrick Sweeney said 0.050 or 50 thousanths was a good minimum. This may vary from pistol design to pistol design but it's a place to start.
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0.050 or 50 thousanths was a good minimum.
If you're lucky, you can get that much without a lot of extra work. Most factory pistols run from .035 to .045 inch... or between 70 and 90% vertical.
More important is equal horizontal engagement... all three lugs taking their part of the load... but few that are not hand-fitted see it.
Ok, here's what I've got. These numbers are averages from several similar pistols.
Normal unlocking distance is 4.7mm but the gun can be made to fire at 2.8mm of rearward slide travel (50% of the locking surface still engaged).
I used a similar approach as yours for determining average bullet velocity in the barrel. My back of the envelope says that the slide travel during the interval that the bullet is in the barrel is 2mm. (Neglecting recoil spring effect and assuming that the frame is fixed.)
2.8mm + 2mm = 4.8mm > 4.7mm
That says that the bullet is probably still in the barrel at the point of FULL unlock. Remember that the figures are averages.
Worst case in the guns I measured was:
3.1mm + 2mm = 5.1mm > 4.1mm
Which would mean that the bullet is going to be in the barrel for another tenth to two tenths of a millisecond. (In the last inch or two of barrel).
My gut feel is that this isn't an ideal situation but that it's not dangerous either.
I gather from your comments regarding the pressure and acceleration curve that you agree...
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That says that the bullet is probably still in the barrel at the point of
FULL unlock. Remember that the figures are averages.
If that happens... IF... that happens... the result will be the brass case exhibiting the "Guppy Belly" syndrome. When the barrel is completely disengaged from the slide, slide and barrel can then separate. Case continues rearward under pressure until it backs out far enough for the area forward of the webbing to become unsupported. Even allowing for the rapid pressure drop due to increased volume in the cylinder, it's highly likely that it would be sufficient to exceed the elastic limit of the brass... or at least be high enough to bulge the case in the thin area in front of the case web.
Someone here who knows what the elastic limits of a typical brass pistol case are... and how much pressure it would require to cause a permanent stretching may chime in and give us some figures.
Here I was under the impression that a designer's job was to make sure the pistol could NOT fire unless fully locked, or at least locked enough not to be dangerous. I wonder why anyone would want to have the gun unlock sooner.
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the result will be the brass case exhibiting the "Guppy Belly" syndrome
Remember, I'm talking about a pathological case--where the gun is somehow induced to fire at the absolute maximum amount of rearward slide travel at which the firing mechanism will still operate. In half the guns I measured this would result in the barrel unlocking prior to the bullet exiting the barrel, but only one of the pistols showed enough of a discrepancy that I consider it significant in light of the assumptions I've made in calculating slide velocity. This is also the gun with the most wear.
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Here I was under the impression that a designer's job was to make sure the
pistol could NOT fire unless fully locked, or at least locked enough not to be
dangerous.
If you take that view of the topic then one could take my results, turn them around and state that with at least some designs it's safe for the gun to fire with only 1.1mm of slide travel remaining until full unlock and only 45% engagement between the locking surfaces.
That would definitely be a valid opinion, especially since none of the 6 pistols I tested have been modified in any way that should change the parameters I'm measuring.
It's also worth noting that my slide velocity estimate is not taking recoil spring force into account and assumes that the frame is fixed (when it actually moves during recoil). More realistic assumptions will drop the slide velocity somewhat. But I'd have to reduce the slide velocity by about 44% to make a significant difference in the worst case I measured/calculated.
If one DID take that view (that the designer insured it was safe), it would be supported by the fact that redesigning a very inexpensive part would ensure that the firing mechanism would be disabled much earlier in the slide travel. In other words, it doesn't seem to be a concern--if it were, it could be VERY easily remedied.
The bullet is gone before any significant un-locking or link-down has started.
If not, the gun would shoot about 10 feet high from the sights at 50 yards when the rear of the barrel started to drop down and unlock.
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If not, the gun would shoot about 10 feet high from the sights at 50 yards
when the rear of the barrel started to drop down and unlock.
In normal operation (and in half the guns I measured even in the pathological scenario I've created) the barrel is not unlocked when the bullet exits. In the normal case, the slide takes about 3x longer to unlock than the bullet takes to exit.
But I'm not talking about normal operation, I'm talking about somehow getting the gun to fire at the extreme limit of rearward slide travel at which the trigger can still be made to function.
Hmmm...
Just thought of another wrinkle based on Tuner's response. The full unlock point may be a red herring in terms of this discussion.
See if this makes sense.
As long as the gun is locked at the point of firing, the slide velocity is going to be per nominal design figures. With that given, the issue (from a safety standpoint) isn't the unlocking point, it becomes point at which the barrel/slide have separated enough to strain the brass. That would easily tack on another millimeter of slide travel which would be more than enough allow the bullet to exit and the pressure to drop.
Accuracy would be totally useless (as pointed out) but the case would certainly hold together--maybe not even show signs of stress.
Check a few Glocks.
Many of them will fire almost out of battery.
In fact, one of my pet theories is that is the leading cause of Glock .40 Kabooms we used to hear so much about.
Especially with lead bullet reloads.
I think lead builds up in the front of the chamber and holds the slide partly open, yet the gun can still fire.
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Many of them will fire almost out of battery.
This discussion is about the definition of "out of battery".
For what it's worth, no Glock I've ever examined will come close to firing with the barrel unlocked from the slide. Pretty much any Glock will fire without the locking surfaces FULLY engaged, but the striker safety and/or the connector will disable the firing pin/trigger well before the gun completely unlocks.
Furthermore, by the point that the slide/barrel have unlocked in the Glock design, the striker is decompressed by about 50%--that reduces the energy stored in the striker spring by around 75%. (Moving the slide rearward decompresses the striker spring). 50% decompression (75% energy reduction) is the level of spring energy seen when the slide is racked but the trigger hasn't been pulled yet--Glock says that's not enough to pop a primer. I haven't tested it, but given that the Glock isn't known for particularly energetic firing pin strikes, it's hard for be to believe that reducing that energy by 75% is still going to provode enough "oomph" to fire the gun. In other words, I believe Glock on this one.
To top it off, by the time a Glock has unlocked, the barrel has moved downward at the breech by enough that the striker can't contact the primer (assuming a small pistol primer) even if it falls with enough force to fire the gun.
With a link down design, the distance over which the slide is locked up in controlled by the lug engagement and the distance between the holes on the link.
On a linkless lock up, the designer has the option to keep the barrel locked up just about as long as desired. The lock up distance is controlled by the distance between the lug on the barrel, the block in the frame that pulls the barrel down, as well as the angles between the two. It makes the issue of controlling out of battery firing much easier.
I know that, but if the slide is even partially unlocked, that would allow even less case support over the feed ramp then if it was fully in battery.
Not much more, but at least a little. (The .004" that broke the camels back? Or blew out the case, as the case may be!)
It would also allow it to finish unlocking much sooner (higher residual chamber pressure) then if it started out fully locked & in battery.
Like I said, it's a pet theory of mine, not a fact!
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Like I said, it's a pet theory of mine, not a fact!
I think that everything you said is a fact with the exception of your speculation on how it would affect the case. And your speculation is as good as anyone's. It is important to remember that extraction doesn't begin until the barrel/slide fully unlock, so you are correct that the difference in case support is very tiny.
The chamber pressure won't be appreciably higher IF the bullet exits.
I'd love to have a lab to be able to test such things.
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On a linkless lock up...
The guns from which the measurements were taken have a linkless lockup.
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Remember, I'm talking about a pathological case--where the gun is somehow
induced to fire at the absolute maximum amount of rearward slide travel at
which the firing mechanism will still operate.
Understood, John... but keep in mind that the 1911... or any tilt-barrel recoil operated locked breech pistol... engages the barrel vertically... but locks horizontally. If even a tiny bit of the lugs are engaged, it's technically locked... and the only way the breech can open is if the lugs shear and allow barrel and breechblock to separate.
Now, if the lugs are only vertically engaged... say bout .005 inch... then the breech will very likely be blown open when the round fires, with disastrous consequences a distinct probability. Think of filing the locking lugs on a centerfire bolt-action rifle down to the point that there's only about .005 inch of overlap with the lug recesses. When you bolt the rifle, technically, it's locked... but if you fired it, you'd probably need a good surgeon to remove the bolt from what's left of your cheekbone.
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but if the slide is even partially unlocked, that would allow even less
case support over the feed ramp then if it was fully in battery
I don't think so, JohnKSa.
If the barrel is locked up enough to keep the slide from blowing back, high enough to let the firing pin hit the primer; then the case is all the way in the chamber with its head against the breechface. There is no lessening of the web support over the ramp.
This is all getting so complicated, I think I will get a Tanfoglio F.A.R. straight blowback so as to not strain my little brain thinking about locking systems.
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...then the case is all the way in the chamber with its head against the
breechface.
What I can say for certain is that the numbers involved would be infinitesimal. On the order of a thousandth of an inch or less.
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the only way the breech can open is if the lugs shear and allow barrel and
breechblock to separate.
And that's NOT going to happen in this case. The area of engagement (even after taking the reduction into account) is approximately 0.027 square inches of steel-on-steel contact at the moment of firing. Nominal (full lockup) engagement area is about 0.06 square inches.
Ok, worst case scenario (in our aforementioned pathological situation) is that the gun unlocks with the bullet about 50% of the way down the barrel.
I need a chamber pressure vs time curve for a typical service pistol round...
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Ok, worst case scenario (in our aforementioned pathological situation) is
that the gun unlocks with the bullet about 50% of the way down the barrel.
Kaboom...
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The area of engagement (even after taking the reduction into account) is
approximately 0.027 square inches
In that scanario, you'd have less than 50% vertical lug engagement.
The barrel lugs will be quickly pounded to the rear... horizontally... creating a distinct "Stair-Step" deformation with attendant flanging on the top front barrel lug corners. Barrel and slide separate by the amount of deformation, creating that amount of excess headspace... and it's excess in the bad direction. As slide and barrel separate, the case backs out of the chamber as far as the breechface will allow. If it reaches the point that case head support gets critical... see above.
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In that scanario, you'd have less than 50% vertical lug engagement.
My measurement/calculation indicates about 46% vertical engagement.
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Kaboom...
I'm thinking that the case won't rupture until its support decreases sufficiently to let it yield. That will take another mm or two of slide motion and by then the bullet will be out of the barrel.
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I'm thinking that the case won't rupture until its support decreases
sufficiently to let it yield.That will take another mm or two of slide motion
and by then the bullet will be out of the barrel.
Depends on what the static headspace was to start with as to where the case would be relative to the point of head support.
But... Let's look at it from a purely mechanical angle.
Assuming that we're speakin' 1911 here... Take the barrel out and insert the slidestop crosspin through the link and swing it into the lug feet to the full in-battery position. Move the barrel straight back 1/10th inch and observe where the slidestop pin is in relation to the bottom of the lower lug. That's roughly how far the barrel moves backward when the bullet hits the muzzle crown. It should still be mechanically blocked from dropping any more than just a few thousandths. That means that the barrel can't link down... and if it can't link down, it can't vertically disengage from the lugs... and it can't unlock.
Consider too the forces involved in the horizontal engagement of the lugs. Let's go back to a bolt rifle. For the sake of argument, let's say that when the bullet is at the halfway point in the barrel, thre's still 50% of the peak pressure slamming the bolt's lugs horizontally against their abutments in the receiver's recesses. If we have a .308 caliber rifle... we're talkin' close to 30,000 pounds per square inch. Think that you could lift the bolt handle vertically against that sort of force? I don't.
Back to the pistol. If the pressure is at 50% of the peak when the bullet is at the halfway point in the barrel... there's 10,000 pounds per square inch of horizontal force against the lugs. Assuming that the lower lug would let the barrel comptely disengage from the slide... think about the load that would have to be on the link and slidestop pin in order to vertically break that contact. Ain't gonna happen.
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Back to the pistol. If the pressure is at 50% of the peak when the bullet
is at the halfway point in the barrel... there's 10,000 pounds per square inch
of horizontal force against the lugs. Assuming that the lower lug would let
the barrel comptely disengage from the slide... think about the vertical load
that would have to be on the link and slidestop pin in order to vertically
break that contact. Ain't gonna happen.
I WANT to believe that, but that would mean a tilt barrel gun CAN'T unlock early under any circumstances unless it actually fires unlocked in the first place or fires with so little engagement of the locking surfaces that something shears. I can't believe the answer is that "pat"...
The linkless design employs an inclined plane to do the unlocking--that provides a mechanical advantage. Think that would help force the slide/barrel apart even under pressure?
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I can't believe the answer is that "pat"...
But it is just that pat.
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The linkless design employs an inclined plane to do the unlocking--that
provides a mechanical advantage.
Nope. The inclined plane only cams the barrel UP. The vertical disengagement is provided mechanically in the exact same manner as the link.
As the barrel reaches the disengagement point, the slidestop crosspin bears against the terminal point... in this case the inner forward radius of the lug raceway... and disengages the barrel vertically.
Place a link alongside a linkless barrel lug and swing it back and forth through its normal range of motion, and it'll start to be less fuzzy.
Tuner, depends on the gun. The FNP family doesn't use the slide stop pin to cam the barrel down. the same block that pushes the barrel up, also pulls the barrel down. There are definitely inclined planes at work there.
On the linkless pistols im familiar with, barrel down is much more abrupt that a linked design.
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I need a chamber pressure vs time curve for a typical service pistol
round...
Sigh, this keeps coming up, doesn't it?
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Nope. The inclined plane only cams the barrel UP.
The particular guns I'm working with unlock by using an inclined plane on the barrel and a matching plane in the frame to cam the barrel down during the unlocking process.
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The FNP family doesn't use the slide stop pin to cam the barrel down.
I can think of a few designs that don't even have slide stop pins.
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The particular guns I'm working with unlock by using an inclined plane on
the barrel and a matching plane in the frame to cam the barrel down during the
unlocking process.
I see... Kinda like the Browning Hi-Power.
Let's take a quick look at that one. The P35 has it all over the 1911 in a couple ways. For one, the slidestop is only a slidestop. It doesn't have anything to do with the engagement and disengagement of the barrel.
That's accomplished by the lower lug geometry and the flat crossmember in the frame.
At first glance, it does provide a mechanical advantage in the barrel disengagement... but not for leverage. It provides a more precise disengagement without the slop associated with a link and pin arrangement, which... in some 1911 pistols, can be as much as .010 inch... and hence the need for links in .003 inch increments. It also negates almost all of any possible barrel "bounce" after it hits the bed, possibly getting back in the way of the slide if the link is too long.
But wait! There's more...
The Hi-Power's barrel moves backward about .050 inch farther than the 1911's before the barrel reaches the beginning of the disengagement timing point. Couple that with a bullet that's 40-50% speedier... depending on bullet weight and operating pressures... and you can see that the bullet is LONG gone before the barrel even gets close to the timing point. Probably a good thing, since the 9mm's operating pressures are almost twice that of the .45 ACP.
So, again... the guns are designed to keep the breech locked until Elvis has left the building... and unless there's somethin' bad wrong... they do it.
Didn't Springfield have a linkless design on one of their 1911s... way back when?
Was it similiar to the Hi-Power?
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Didn't Springfield have a linkless design on one of their 1911s... way back
when?
Don't know. If they did, I don't remember seein' any.
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Was it similiar to the Hi-Power?
Nope. The linkless 1911 barrels that I've seen all worked off the slidestop pin... in and out of the slide. The Hi-Power's slidestop pin only functions as a slidestop. The engagement and disengagement areas of the lower lug operate off a flat crossmember that's positioned behind the slidestop.
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The Hi-Power's slidestop pin only functions as a slidestop. The engagement
and disengagement areas of the lower lug operate off a flat crossmember that's
positioned behind the slidestop.
From your comments, sounds like a better way compared to the link or "linkless" 1911s?
Better would mean more durable, reliable, easier to produce!
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Better would mean more durable, reliable, easier to produce!
Much more durable and abuse resistant whenever somebody plays "Drop the Slide" on an empty gun often and repeatedly. Production-wise... probably a bit more tedious to produce because of closer, more precise tolerances in getting it timed just so. As with any other design, it's essentially a compromise. There are advantages and disadvantages to each one. The biggest problem that I see with the linkless system is that it's not as readily adjustable... more tweak resistant if you will... than the old standard falling link.
I recall reading about the "linkless" Springfield. It had a cam block set down in the frame and I think the only function of the slide stop pin was to hold it in place. It was very similar to the Peter Stahl system - which Springfield also sold for a while - except that it used a standard slide. Most of them were 10mm on the theory that the cams could be cut to increase the locked travel for the heavily loaded original 10mm Norma.
I once had a gun modified for the Jarvis Kam-Lok system that operated like that. Cut a huge mortise in the frame and set in a frame cam block, then fit up a barrel with camming surface instead of link lug. The cam block was retained by the slide stop pin but the slide stop played no part in the lockup. Unfortunately I bought it in 9mm P and the system was so "stiff" that it took stout loads and removal of weight from slide and barrel to get it to work. Wasn't very accurate, either. So I cashed in a favor at a machine shop and had a filler plate made so FLG could convert the gun back to normal operation with a new barrel.
Briley once showed a cam conversion that did run off the slide stop pin. Their idea of cam location required redrilling the slide stop pin hole rearward and use of a purpose built slide stop with a shorter lever.
Doesn't sound like going "linkless" would really be worth it!
Assuming a pressure of 10,000 PSI in a .45" bore, the longitudinal force trying to separate the barrel and slide is about 1,598 pounds. This is the force the locking lugs must resist. Further assuming a .16 coefficient of static friction (steel on steel, lubricated) the force (normal to the locking force) needed to initiate disengagement will be about 256 pounds. This is true regardless of the depth of locking lug engagement. Depth of engagement affects only the timing of the unlocking, the degree of wear of the engaged surfaces and the nature of the force applied to the surfaces; as the depth of engagement decreases from 100% the force seen by each lug changes from a longitudinal shear force to an angular force that tries to rotate the lug around the base of its unloaded surface, while the base of its loaded surface sees an increasing tensile force.
Under normal conditions, at zero pressure after the bullet has exited the barrel, the link need only counteract forces resulting from interference between lugs, and is aided in this endeavor by the weight of the barrel.
The only way to arrive at this 10,000 PSI unlock situation is for the gun to fire out of battery. If the bullet has moved halfway down the barrel to reduce the pressure to 10,000 PSI, then the slide has also moved backward due to recoil, and might have enough of a running start for its momentum to yank the link hard enough to unlock the action. What is certain is that as the bullet moves further down the barrel, reducing the pressure below 10,000 PSI, the slide gets a better running start and eventually reaches a point at which unlocking is sure to occur, abeit at a still dangerous pressure level.
I do not completely remember the mechanics of how the 1911 operates, but vaguely recollect that one of the holes in the link fits its associated pin loosely. This would allow a yank on the link to easily pull it past vertical, at which point it would would start to develop a down force. My recollection could be B.S.
This entire post could be B.S.
Good post HammerBite, and that sounds pretty accurate. One tiny point...
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Depth of engagement affects only the timing of the unlocking
You may be confusing "Time" with "Timing." Time is a function of distance, while timing is mechanically fixed. They're related... but they're not the same.
Assuming that your car's ingition timing is set at 10 degrees BTDC... and assuming no automatic spark advance... the plug will fire at 10 degrees BTC whether the engine is idling or at redline. That's timing.
The time lapse for the piston to travel from BDC to TDC will vary according to the engine's rpms. That's time.
If the lugs are vertically deeper into the slide's recesses... it will require a longer distance of barrel drop to clear... which means more time. The event will begin at the designated point regardless of slide and barrel speed... nominally at about .100 inch of slide travel.
Tuner, the cam block machining is relatively simple, and doesn't require a whole lot of precision, because the designs allow the barrel lugs to travel well forward. With a link, your forward travel is more limited.
The Hi-Power's slidestop pin only functions as a slidestop. The engagement and disengagement areas of the lower lug operate off a flat crossmember that's positioned behind the slidestop.
True that the BHP slide stop plays no role in locking and unlocking, but it does have another function - it holds the gun together.
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If the lugs are vertically deeper into the slide's recesses... it will
require a longer distance of barrel drop to clear... which means more
time.
That is what I was referring to.
Question: Are the load bearing surfaces of the locking lugs straight-up vertical to the bore center line? If not, which way do they lean?
Another question: Is the link straight-up vertical at full lockup? If not, which way does it lean?
Yes. The lugs are vertical, thought the barrel tilts up when they're engaged.
As the gun is fired, they deform slightly and "seat" to one another, with the forces bearing on the barrel lugs in slight compression... which makes'em a little more shear resistant than straight line. It's very slight, and within a couple hundred rounds, the lugs will "seat" and wear in... and hopefully equalize. That depends to a large degree on how close they are... slide to barrel specs... tolerance stacks, etc.
Back in the "olden" days... when slides and barels were softer... this was accomplished by firing a few proof loads in the gun. Nowadays, with the harder steels, the pressure-seating operation isn't as viable... nor as fast.
Lugs crack instead of deforming... so less intense force spread out over more time is the best way to seat the lugs. The popular "Break-In" period. Of course, it depends on how closely they're fitted to start with. If one match- up has too much distance between slide and barrel lug... that set may never touch within the life of the barrel.
No. The link is slightly past dead vertical when the gun is in-battery by some few degrees... or at least, it should be. If it weren't, the link would start to exert a downward influence on the barrel almost the instant the slide moved.
CAUTION: The following post includes loading data beyond currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The High Road, nor the staff of THR assume any liability for any damage or injury resulting from use of this information.
This is a 2007 Kel-Tec P3AT in .380 with locked breech.
I shoot it with the hottest load it is can do: 6.2 gr Power Pistol, 90 gr Gold Dot in a locked breech
This is 1907 Husqvarna copy of a 1903 Browning blowback in .380.
I shoot it with the hottest load it can do: 8.5 gr Power Pistol 158 gr .357" XTP.
That load is more than the Alliant maximum load for that bullet in 357 magnum
Link to Alliant 8 gr 158 gr 357 mag load: Alliant Powder
Who needs a locked breech?
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Who needs a locked breech?
All ya need is a stout enough spring or a heavy enough slide... and a willingness to blow up a gun, I guess...
The Husqvarna started out as a 9mm Browning Long, chambers sleeved to .380 to sell surplus in the New World. Dean Grennel wrote that it took heavy loads developed in a locked breech .380 to function the converted blowback and that P&V testing showed them right up there with 9mm P. Which makes sense, the 9mm Browning Long (9x20SR, you could make brass by shortening .38 Super.) is 90%+ of SAAMI 9mm Para ballistics.
Here's an example of a 1911 barrel with insufficient vertical lug engagement. This one checked out at about 50%.
Note the stair-stepped setback on the first lug wall. This means that the slide and barrel separate by a like amount whwnever the gun is fired. As the deformation progresses, the amount of separation grows... and so goes the headspace. Endshake... endplay between barrel and slide likewise increases, which makes the deformation self-perpetuating and accelerates the process due to the lugs having a longer "running start" at each other.
The headspace increase is in the kaboom direction because... as the headspace increases and separation of barrel and slide occurs... the case backs out of the chamber as far as the breechblock will allow. It eventually reaches a point that case head support is critical. Study the picture, and it'll start to make sense.
