I have been reading as much as I can through the 'search' feature regarding uniform barrel lug engagement to the mating recess milled into the slide.
The problem that I seem to have is in not understanding what I read. I 'spect there is as large a difference in 'reading' and 'understanding' as there is in 'looking' and 'seeing'.
I am curious as to how the amount of engagement surfaces are measured, as well as how the amount of lug engagement to the slide is corrected/adjusted.
If my convoluted brain synapeses are firing in the correct direction this morning, it would seem that if there are minor 'drag' marks on the barrel in the area above the chamber, that the barrel lugs are engaging the mating recesses in the slide uniformly.
Whew! Dang Salty... That's a tall order for a write-up. That's one of those things that I'd rather explain face-to-face than try to write it. It's gonna be long, so I'll do it on a notepad, edit and shorten it as much as I can, and then cut and paste it on the reply. I do that a lot anyway.
Real quick, though... a few things to think about while I get it ready.
The lugs are fitted vertically for depth and overlap. Think of the cross section of gears in mesh, and it'll be clearer... and horizontally. Very few factory barrels have full depth OR equal horizontal engagement. With most pistols, you're lucky to have two lugs bearing in the horizontal with more than 75% depth/overlap, and that includes the revered USGI pistols. If you're very lucky, one of those will be the first lug...the strongest one. My minty Rand bears on the forward two, with the first lug kissin' about .006 inch of air, with about a 90% depth and overlap. Good... but not as strong as equal engagement with 100%. Strong enough for its intended purpose though, the .45 ACP not being a very intense round, pressure-wise.
Horizontal equalization of the lugs... or having all three bear an equal share of the recoil forces... requires measuring the distance from the breechface to the forward wall of each lug in the slide. That calls for a gauges, as each one is measured individually. I sold all my stuff off years ago when I took my sabbatical from the practice... and had to make a set when I started foolin' with it again. My gauges are simple. I made'em from old, take-out barrels by removing the lugs that I didn't want to use for a given slot in the slide, and leaving the one that I did want. Three barrels required, plus a set of feeler gauges. (One day, I'll modify the barrels to adjust directly onto each slot, and won't need the feeler gauges. Right now, lack of time and laziness delay that operation.)
Vertical engagement... or lug overlap... is largely determined by the variations in barrel lug height and slot depth... along with lug and slot width... and it does vary quite a bit from slide to slide, barrel to barrel. Standard dimensions have tolerances. How things fit depends on which way the tolerances stack up in the vertical plane, with the slide, both upper and lower barrel lugs, and frame's dimensions all coming into play...including the location of the slidestop crosspin... The height of the frame rails... thickness of the slide ways all figure in.
In the horizontal plane, narrow barrel lug and wide slot mean a loose fit Go the other direction, and you may need to alter things a little... Some file'em and some machine'em. Either way will work, but using a file to narrow down a barrel lug requires patience. Cutting the rear face of the lugs isn't as critical as the front faces, because the front faces bear the shock of the recoil, and have to be as square and true as possible to minimize time required for them to seat and ewqualize under pressure, and also to prevent sideways flexing of the barrel when the gun is fired during the seating process. Machining, with with the set-up done with a dial indicator to keep things square is probably best when given a choice, but a skilled man with a file can do as well given enough time. I use a file... carefully.
In the days before there were so-called hard-fit barrels, the lugs were welded up and recut... by hand or machine... to fit the mating surfaces in the slide, and much fit-and-check was required. A slow process, and not really the best. Welding alters the grain structure of the barrel steel, and unless the welder really knows his business, the smith doesn't have a clue what charactistics the steel will show. Might be softer... might be harder to the point of being brittle. The aftermarket gunsmith fit barrels come with oversized dimensions that cover all bases, and can be whittled to fit all but badly out-of-spec slides. Again... due to varying tolerances in the slides, some will produce better results than others, but generally, any slide that is fairly close to spec can produce a top-grade fit.
Years ago, when barrel steels were a bit softer than today's, the lugs were typically proof seated by having the first lug bear the brunt, and sometimes the second one was kissing, leaving the forward lug with about .002 inch of clearance. Proof-firing with overpressured ammo... part of the acceptance process... usually would set the engaged lugs back the required amount to get the first lug into play. It wasn't necessary for the first lug to bear a completely even load at this point, as things would continue to seat as the gun was fired, until they equalized. At that point, further lug setback would be so slow, that the barrel would likely be worn out before the lugs were battered so far back that headspace became a concern. Vertical depth of engagement and overlap also figured in. The more the overlap, the stronger the lockup and the slower the lugs would set back. Also, the more end play between barrel and slide, the faster the lugs would set back. Known as "Slap- Seating"...a barrel that had .010 inch of horizontal play would get a running start as the pressure came up on firing, and would hit harder than if there was no play, which would apply horizontal force on the lugs more slowly, as the pressure curve started. Simply put, the more slop, the higher the pressure would be when the lugs whacked each other. Think of pushing against a wall with your fist as opposed to punching it from 10 inches away.
Even if the same total force in foot-pounds were applied to the wall... which one is gonna hurt more? As the lugs set back, headspace increases because when the gun is fired, the barrel is held forward by pressure and bullet friction, while the slide is forced rearward through pressure slamming into the brass case. The case doesn't have anything to hold it forward, other than its own friction against the chamber walls, and it pushes the slide backward until the lugs bear against the barrel lugs. That means that the case backs out of the chamber by the amount that the slide travels under pressure... or total working headspace. A certain amount is allowable... More than SAAMI maximum means that too much of the case head lacks the support of the chamber, and a blowout is possible. Working headspace increases with use.
How quickly it reaches the maximum allowable is largely determined by how much it started with... how closely the barrel is fitted... and what type of ammo you typically fire. Higher pressure means faster increase, and vice- versa.
Remember too, that the 1911 isn't a true locked-breech weapon in the strictest sense...n ot in the way a bolt-action rifle is. It only becomes a locked-breech when it fires.
Any place for PlastiGauge?
Reminds me of stories about getting all the lugs in the Weatherby Mark V to bear (proof loads until done).
I read about using modelling clay and such, is there any place for PlastiGauge such as is (or was when I was into racing) used to measure bearing clearances - it seems to me ideal to replace clay for at least vertical clearance. Then too I read about fitting barrels with sand clearances and such - usually at extra cost if the customer wants to specify such.
I've used plastigauge with good results... I just haven't used any lately.
I've also used modeling clay to check depth, but it's a pain. Even if ya coat things up with graphite, it still sticks about half the time, and the graphite has a thickness too, so it'll give a (tiny) false reading. Not enough to make any real diffeence... but it's there.
You can still get plastigauge through auto supply houses and probably any automotive machine shop should have some on hand. You may have to order it, though.
I think the Plastigage application is ingenious, I have a question, though.
As used to determine bearing clearance, the instructions always stressed the bearing caps had to be torqued to full value. Might not the elasticity in the Plastigage give false readings between 2 surfaces that are not torqued to 110 ft-lbs?
Thanks for reminding me how high those 4-bolt mains torque values are on the BBC.
Plastigauge gives a pretty good ballpark estimate, but isn't as precise as modeling clay. It's just easier to use, and saves time while ya get close. The final measurement should be made with clay, but if you're not trying to get the nth degree of accuracy out of the gun, plastigauge will do. A little trick that helps is to assemble the slide and press the barrel in with a toolmaker's clamp or C clamp, and warm it in the oven at the lowest setting for 30 minutes or so. Remove the barrel and mike the strip before it cools off.