It's late and I'm getting confused maybe. But why do NM specs call for a barrel hood to Lug One as being .017" longer than the same space in the slide? I'm measuring a slide and barrel and just noticed this on the "base" NM measurements. Are the measurers expecting .017"+ to be cut from the hood on fitting? Then why do they not simply include that (cut) in the overall length?
Hi David: Well, it's early here and I'm already confused!
The NM Slide blueprint shows 1.317 + 0.003 inch as the breechface to lug#1 distance, but i don't have a blueprint for the NM barrel.
All my NM data is from an article in the April 1963 American Rifleman in which the author writes "The hood has been left sufficiently long to insure that it may be fitted to the breech face of the slide even though some setback may have occured in the locking recesses."
These NM barrels were made available through the DCM for fitting in non-NM slides by civilian shooters, so maybe that's why the hood spec seems so big.
This article has lots of chamber plus other barrel and slide data, but nothing for the hood to Lug #1 distance.
What's the source of your NM barrel data?
I *thought* the data came from Kuhnhausen Vol. 2. I have it printed on a spec sheet that I fill in with numbers off my build for the slide and barrel. But when I looked it up just now, I found the difference to be only .005, not counting tolerances. HUH? No telling where I thought I got it... Or maybe things just got miscombobulated. Did I spell that right?
But that means that my Kart NM barrel is even farther out than I thought. It measures 1.330. Of course, the bottom line is how it fits the slide. The Caspian slide that it is going into is dead on the NM specs. So there is a lot of whittlin' to do. Most of it can come off the hood thankfully... but not all.
I'd be really close on lug one and two, but lug three is "out there". What is the depth engagement on number three?
Do you mean Kuhnhausen Vol. 2 has the NM barrel specs? If so, guess it's time I broke down and got one.
The height of the slide's #3 lug wall works out to 0.0099 less than the #1 lug wall. As I'd figured the #1 lug wall was 0.0612, that'd make #3 lug 0.0513 inch high. Mid-spec values for all, as usual.
He has some at least of the NM specs in Vol. 2. There is some redundancy, but I've not really sat down and compared them. I grab whichever I remember had what I wanted. Yeah, fork over for one. It's a worthwhile addition to the library.
Thanks for the calcs. #3 bears considerably from these measurements. I know that a lot of folks don't even get #3 close on factory guns, but this one isn't "factory", so... But I've discovered a potential slide problem, so if anything is done about it, I may have a new set of numbers to deal with anyway. I won't post anything about this until I know what Caspian will do. That isn't meant to be a "teaser". I should have looked it over better. I hope to post kudos. We'll see.
Quote:
But I've discovered a potential slide problem...
Is it that lug #2 is not exactly half way between #1 and #3?
I'll try to list this in a sensible way:
Barrel
lug 1 to hood = 1.330
lug 2 to hood = 1.653
lug 3 to hood = 1.978
Slide
lug 1 to breach = 1.316
lug 2 to breach = 1.642
lug 3 to breach = 1.975
So the difference is over on the barrel lugs by
1 - .014
2 - .011
3 - .003
So to get "equal fit", I would need to take .003" off the hood, .011" off lug 1, and .008" off lug 2. Lug 3 would be OK. But that is a lot of metal to remove with hand tools uniformly. At least it is for me. And that says nothing about file cost.
.011 off the hood would allow 0 off lug 2, .003 off lug 1. But that would leave lug 3 hanging in the air by .009".
See the dilemma?
I've not set up in a lathe as Tuner once said for scraping. But I may need to do this one. The problem is that you can only cut a very limited amount of the upper part of the lug with a 3-jaw chuck. My 4-jaw weighs a half-ton (for an old guy), and I hate to mount it. But like I said, this may be one of those times.
Does anything show the radius of the lug cut themselves, to see if this is even feasible? I haven't looked yet.
Hi David: Don't think of it as a dilemma, but as a opportunity to rise to the occasion and conquer!
The barrel seems OK based on your figures and my USGI (not NM) barrel blueprint. Lug wall separations are within 0.001" of the mid-spec values and you certainly have plently of extra material in the hood.
The slide is within the USGI specs, but lug #3 is 0.003" forward of the NM upper limit.
Maybe this is one of those occasions where contact at the first two lugs is all that can be obtained.
The barrel cut radius and center (actually, the arc) locating data are on the blueprint in our Tech Issues section.
If the barrel's not short-chambered, how much forward can you go with the barrel without running into headspace problems?
I don't remember how much room I've got for headspacing, but I finally forked over for a reamer. Now YOU fork over for Kuhnhausen Vol. 2!
I'll check those specs.
The reason I simply referred you to the blueprint is there's one digit in radius spec on the blueprint in our Tech Issues section that's pretty fuzzy to me. Didn't want to mislead you.
Hopefully you've got better eyes, monitor or printer and can get it deciphered correctly - or rely on Kuhnhausen (which I will get some day).
2 and 3 lugs have a .348 radius -.005. 1 has a .355 radius -.005. This is from Kuhnhausen Vol 2 for measurements on the NM barrel.
Thanks Iron Bottom. Still haven't taken the time to look them up.
I hadn't thought about them changing the radii (?). Another setup! It might just be easier to take a few careful file strokes to get the one on down. If I end up cutting all three lugs for contact, setting up on the lathe would probably be worth it. Though I've never tried scraping on the lathe...
Does anyone have an idea how heavy a pass these scraping strokes should be? Will tool steel be better than carbide for this? My suspicion is that carbide would be over stressed. Tuner, are you following this? Or have any followers done this?