Went shooting with an AR i built this weekend. Had undergassed issues before but ended up being an extremely leaky new BCG. Got a replacement for it and all is well as far as undergassing.

Now, I shot about 250 rounds this trip out, and had 2 or 3 instances where the bolt would lock up TIGHT on a live round in the chamber, I believe fully forward into the locking lugs, but not fire. Took all I had to extract the round via pulling back the charging handle. I can't remember if they were only on reloads or some factory american eagle also, but other than ammo issues, is there something that could cause this? I also had one FTF (looked like a primer issue, had good strike on primer, but didn't go), and a few slight hang fires where there was a noticeable lag between hammer pull and firing (maybe it being 20 degrees and using a ball powder had something to do with it).

I did take one of the extracted rounds, put it back into mag, charged the weapon and it shot/extracted without issue the second time. So I'm a bit at a loss..... I don't pretend to be an AR aficianado, though I am pretty knowledgeable at this point how they work.

On bolt guns I'd think that the bullet buried itself into the rifling, but being chambered 5.56 and loading to less than mag length (2.229 OAL) with 55gr FMJ's, I'd REALLY doubt that is happening. This is a new BCG from stagarms that replaced my leaking BCG. I'm going to buy a bunch of factory ammo and shoot it and see if it happens to factory ammo just to really see if it happens to that also.... but I think it did once.

Now, the accuracy was awesome for the components used (24.9gr of TAC with hornady 55gr FMJ-BT). Should be better once I get some 62gr's loaded up as the 1:8 barrel should like them a bit better. I couldn't be happier about that, but this issue has me a bit concerned. I am going to go over my reloading checks once again to see if I can find anything. Though these were sized/primed by a friend of mine. I know my QC checks are pretty rigid, unsure about his.

Was this weapon head spaced .

No, but I guess I can go get it looked at by an armorer. I guess I fell under the 99.9% of the time if using good components you shouldn't have an issue thought that is pretty prevalent.

This has a wilson combat barrel and stagarms chrome bolt carrier group.

I noticed you were shooting with a combination of reloads and factory; try it again with facrory ammo only.  If the reloads were not resized specifically for your chamber or if the bullet jams in the leades it places the bolt in a bind agianst the barrel extension.  Once you have slammed the shoulder of the round into the chamber it effectively resizes it slightly giving you a better fit the second time.

Yeah, I did check quite a few with headspace gauge, but when I reload I check nearly all to ensure all is setup right. Like said, this wasn't my resizing/trimming/priming. I made sure my stuff is set up right, and it seemed like his stuff was OK, but maybe he had some issues with some brass.

I'm using a small base die which from what I gather bumps the shoulder back about .002 more than standard dies. I don't think this should be of issue. The brass I have resized passes headspace and I use a worlds finest trimmer set to 1.751 to uniformly trim, then do the usual powder drop, seat, and crimp (very light using FCD).

I'll buy 200 rounds of factory and run it exclusively next time at the range and see if I have any issues. If not, it would be obvious the issue lies in the loads. Then I'll shoot my reloads exclusively, or maybe even make some dummy rounds (no primer/powder), and hand-rack them mag after mag to see if I can replicate the failure.

Steve

What did you do with your old leaky BCG?

requested a replacement from the manufacturer, and got a good one in return (thank god). Stagarms handled it without question and quickly once I got it figured out.

Well, I bought 200 factory rounds to burn through the gun shortly (its cold as heck right now and snowing), see if this situation presents itself again. It does seem that my reloads are noticeably longer than factory FMJs (though i have hornadys, not too sure what they are loaded with). Maybe just maybe it is long and due to some bullet variances, it buried a couple into the rifling. I doubt it, but maybe. The other thing I'm wondering is if maybe case expansion occurred near the web, I'll try to take measurements.

I'm going to plop one round that I know jammed up into the headspace gauge to see what there is to see. I'm also thinking about loading up 20 dummy rounds with my setup just to see if i can replicate the issue.

I had this issue with my rifle too! Mine was I sent my BCG that came with the rifle to get nickel boron coated. When I got it back, when shooting with downward pressure on rifle (shooting with bench rest), it would chamber the next round but won't fire. I would have to use a wooden 2x4 to pound the charging handle to eject it out. My range owner is a big retired cop and he couldn't even rack the charging handle. Anyways, the ejected round would have some serious shoulder damage and light primer strike but didn't go off. I could load it back into the rifle and fire it no problem.

I found that my rifle would function fine when I put the pressure backwards into my body. So pretty much, I had to run 100 rounds to break the BCG in. Now it works fine and hadn't done it since! (Jinx).