The other day I tested out my new build. BCM 12.5" MCMR upper with BCM bolt. Aero Lower with Geissele trigger and LPK.

I cleaned and lubed it all up then got to shooting. On my 3rd round the bolt was stuck solid almost in battery with a live round. Myself, 2 RSOs, and a gunsmith couldn't get it unstuck after screwing with it for about 10 minutes. Mortaring it on the ground didn't seem like a good idea being at an indoor range and with the brace I had on it. The 3 of us spent 10 minutes or more trying to get it unstuck before they gave up and I kept messing with it gently trying to jiggle a flathead screwdriver to force the bolt back. Finally a combination of forward assist slaps with trying to separate upper and lower and it came apart.

I was shooting my reloads, but I took the stuck round out and checked it top to bottom. No different than any of the others that have worked fine. After that I put another 100-200 rounds through and had no issues. What could have caused this?

Out of spec reload.

Neither of those fit though. I collected my brass and the primers were fine. The stuck reload is identical to all the other rounds.

Almost in battery is the clue.  The body of the cartridge was just a little over diameter and the taper of the body locked up on the chamber wall.  If the shoulder was too far forward it would have been easy to extract.

It firs into the case gauge perfectly like all the rest. Like I said I calipered it and checked it too to bottom. No different.

I was expecting to get home and check it to find it hadn’t be resized or something. It seems no different.

As for why I didn’t fire- I Pulled the trigger and got nothing. The primer possibly has the tiniest mark from the firing pin. I mean barely noticeable.

Interesting. I wonder how that could have happened. I’d say I somehow missed resizing it but it wouldn’t have been decapped if I did.

RCBS 5.56 die. Dillon precision case gauge.

Should I be worried about the rest? I’ve shot about 500 of these reloads with only the one hang up, and I still have 500 more. This is gonna bug me...

Could have been you pulled the handle far enough to decap but not far enough to resize all the way to the base.

It's happened before but I'm going to have to plead the fifth.

If these rounds are not for serious use, just use them and do remedial action when one doesn't chamber.  If you want to avoid the remedial action you can get a gage to check the rounds before you use them.  Any rounds that don't check out you can pull the bullet, save the powder and bullet, adjust the decapper/ neck expanding ball so that the ball enters the case fully but does not decap.  (Be careful you do NOT want the expanding ball inside the neck when the neck is reduced in size by the die.)   Then drop your powder back in and reseat the bullet.

If you don't want to buy a gage, you can dissassemble your AR and manually drop them in the chamber and check that they fully seat.  More hassle but more cheaper.

I'd guess that since it fits in a case gauge, the load was good.

You could have gotten all kinds of junk in there just right. Althought I'd half expect that if it locked up that bad, the case would get dented from the junk.

It is a short barrel, so it's likely dirty as sin, especially if you used a can. My rifles lock up bad if I don't use a liquid oil after a couple hundred rounds with a can. The powder fouling turns grease into glue. Usually locks up around when the bolt starts into the barrel extension. Get it cleared, cycle some clp or motor oil in there, and it runs for hundreds of rounds.

I haven't done a torture test like that because of climate, too humid and will rust. Also not trying to shoot 2 cases in a day, but I know my one rifle will go 1500+ without cleaning over a few months. It's reliable... unless you mix grease and suppressors.

Maybe you popped a primer and didn't notice it falling out.

Maybe just write it off as a fluke.

I loaded 100 round when I first started reloading and didn't understand "cam over" on the press. I didn't bump the shoulders enough and it did exactly what your describing.

I just had a buddy go thru this with his 6.5 cm...his brass would fit his case gauge but would not chamber in his rifle...I had him screw his die down a 1/8 turn and brass than fit rifle fine...wasn't quite going down far enough to size every case enough to chamber...

Out of spec reload.  It may fit the case gauge but still be out in your particular chamber.

Also, keep a rubber mallet with you when you go shooting.  Pull on the latch side of the charging handle by hand, while you tap on the opposite side with the mallet.

Bad Ammunition.