Cracked bolt replacement issues

SWANEEDMB wrote: How many carbines if anyone knows blew up due to a bad bolt? Sure, have heard a failure due to some ones reloading but never a bad bolt. Really have enjoyed this thread.

I have measured/compared a lot of bolts and have never found more than .001-.002 difference between any of them, flat or round in the places that matter. That is likely the allowable tolerance. I will say that I surely have not examined as many as a lot of members have here, but I am yet to find one that varied to any serious degree. I think most of the problems (ammo aside) with exploding carbines are receiver or chamber related. Receivers stretch with use and chambers wear for the same reason, use and maybe abuse. Notwithstanding cracked/broken bolts, or serious lug wear on contact surfaces, I doubt many bolts cause blow-ups.

I must say (and I think just about everyone else will agree) stop feeding the gun the Tula. May have nothing at all do do with your problem, but could cause problems on it's own. I have not read-thru all the former posts, but did you ever make an attempt to measure the head-space? We know the gun was re-barreled, but maybe the cracked bolt was just close enough to allow a firmer firing-pin strike B4 it broke. If the head-space is long, some rounds may fire and some not. It gets down to the thousandth's of inches in difference between any particular case length. If the cases are "blown-back" against the bolt face, more of the case is left unsupported and could be the cause of the bulging you say you see and possibly could have had something to do with the first bolt cracking. I don't think you can determine much of anything until you know the head-space.

Em...in my learning days, I had the exact same experience with a carbine, until I figured it out. The gun would sometimes misfire and deliver light strikes and when the very same round was loaded again, it may fire right off. I also cracked a bolt and eventually determined there was a small burr under the left-side receiver lug recess that I had not completely cleaned-up. ANY slight drag or interference at all will lead to incomplete bolt-rotation and result in light strikes as well as a cracked right bolt lug. When the slide slams against that right lug, something has to give and any interference in smooth bolt operation will cause it. I also discovered that with the bolt I was using, the head-space was very excessive. When the round ignites, the case starts to expand and grabs the chamber walls. This action in-and-of-itself tends to hold the case in place against the chamber and somewhat reduces the rearward "slam" of the rim against the bolt face. If head-space is severely loose, the bolt lugs take the brunt of the force as the case moves rearward. Since you are concerned with the way the cases have expanded, this could be caused by the case moving rearward and expanding into the larger chamber dimension toward the rear of the chamber. The chamber is tapered, being wider at the rear. If the round is allowed to move rearward due to loose head-space, the brass has to expand further to grab the chamber walls.

In my case, finding a bolt that properly head-spaced and making sure all the rubs at contact surfaces were clean and free of burr's solved my problem. The gun may not have been properly head-spaced when it was re-barrelled and you'll not know unless you precisely determine the head-space. Not just with go-no-go gauges, but with a case of known length using an accurate measuring tool. It's been explained here B4, but if you want to PM me, I can tell you how to determine what the exact head-space is. I think that is what you have to discover B4 you can begin to diagnose and correct the problem. It sounds to me like the gun was never properly head-spaced when it was re-barrelled?

Obviously I would prefer not to use steel cased ammo but it was more or less a test to see what ammo functions/fails since I am having trouble getting it to work in the first place. I haven't measured the headspace but if it is in fact excessive (based on the bulged brass) then I assume the next logical step would be to bring it to a gunsmith?

https://imgur.com/a/Zth8s

Here are some pics of a round seated. It sticks out about 1mm from the edge of the receiver, or about 5mm from the edge of the barrel. As for movement it seems relatively well seated but I could discern a small amount of "jiggle" if I put my finger on it and try to move it.

Em one,

manteo97 wrote: From those photos showing "stick out" of the cartridge, plus the bulging, you have a short chambered barrel on this carbine. It was not reamed to proper/safe headspace. If your 5 mm measurement is accurate, stick out is more like 3 mm on a proper depth ream job, that's about .080" difference (sorry about mixing metric and imperial) which is near the dimension newly made barrels are short chambered too. If it's a Plainfield, they did not put skirts on their original barrels, and this skirt should have been removed before installation. Don't shoot any ammo through this anymore until a gunsmith, that knows M1 carbines, has worked on it. Ask if they have correct barrel clamp and receiver wrench, proper finishing reamer ( preferably a pull thru type), and GO NO/GO gauges. May be fixable.

For what it's worth coming from me.....this gentleman knows what he's talking about. It could be that OP could get lucky and a properly reamed chamber could fix this gun? Tight head-space may allow the bolt to close and rotate...barely on brass that is short enough and not fully close on slightly longer brass and this condition will surely crack a bolt at best or blow-up a carbine at worst.

Good Luck!

I'd say that if he can fix the gun without barrel removal, let the skirt alone. I wouldn't  pull the barrel just to remove the skirt.

Just had a few more thoughts:

em-one, if you have a vernier caliper, and have some unfired rounds from the same lot as those bulged fired brass, check the length of the fired brass vs. the unfired brass. See if there's any difference.

Also, I had a look at my 30 carbine chamber reamers, and the rougher has no cutting edge for the throat, unlike my finishing reamer. If I was in the barrel making business, I'd make short chambered ones, using only the finishing reamer.

So, this could mean that your short chambered barrel has not had the proper throat machined out so the bullet can't seat correctly before firing.

manteo97 wrote: Just had a few more thoughts: em-one, if you have a vernier caliper, and have some unfired rounds from the same lot as those bulged fired brass, check the length of the fired brass vs. the unfired brass. See if there's any difference. Also, I had a look at my 30 carbine chamber reamers, and the rougher has no cutting edge for the throat, unlike my finishing reamer. If I was in the barrel making business, I'd make short chambered ones, using only the finishing reamer. So, this could mean that your short chambered barrel has not had the proper throat machined out so the bullet can't seat correctly before firing.