The dents you see are from the lugs on the barrel extension, the rounds were jamming in the upper at an angle.
I should add that this rifle has functioned flawlessly when shooting XM855 and using Magpul/Colt/Okay magazines.
This time, I was using an HK magazine and was shooting Lake City ball ammo.
I should also add that this same magazine and ammo fed fine out of two other rifles on the same day, for some reason this rifle did not like this ammo/magazine combo.
Edit. What did BCM say? Call them first, but if you want to troubleshoot yourself:
Ive seen that from a bent extractor.
Change the bolt, if fixed, put the old one back in but swap extractors with a different rifle. Or lat the extractor on a flat surface. I suspect a bad extractor spring could also cause this.
Could be mags. What I think is happening is that the round being fed is getting loose before or during the feeding process.
In the case a saw, the extractor lost control of the case, it bumped the top round of the mag knocking it loose as the bolt was trying to feed it. The empty case then exited the port.
Did you use the XM855 on same mag? Some mags just don’t work well with different ammo. For example, one ammo I used did well with standard mag but did not like Magpul or Lancer mags. The problem it seems stems from the feedramp… Something is happening there that may be hitting the bullet in the chamber once it is seated when bolt is racked forward. It could also be the way the bottom of the casing of the particular round that like city ball is using. It might be loose enough to cause a slight angle when bolt is going forward.
…the extractor lost control of the case, it bumped the top round of the mag knocking it loose as the bolt was trying to feed it. The empty case then exited the port.
Typical of an extractor issue, resulting in the damage shown on the cartridge.
Sell the HK magazine. Take the 30 bucks and buy 3 Okay aluminum magazines. It’s probably not contributing to the issue but I would feel better with the M4 ramp cuts in the receiver. It isn’t difficult or expensive to do.
I didnt know HK mags had a rep, I thought they were all good.
Absolutely could be a mag issue(mechanically). Mark the mag, keep shooting, track malfunctions, and if its always that mag, trash it.
Hard to say but this could also be the result of short stroking, where the bolt lugs don’t travel rearward enough to clear the rim, then as the bcg travels forward the bolt carrier catches the rim instead of the bolt. Then the cartridge gets wedged in there with the feed ramps putting the dents in it.
I would guess he didn’t call BCM and even if he did contact them, it’s Friday and we all know no one is going to answer until at least Monday or Tuesday. In addition, though he is using some BCM components, he has clearly assembled this with a non-BCM upper.
I own a lot of magazines and the numerous HK magazines I own work just as well as the aluminum GI mags. I don’t shoot a lot of LC ball out of any magazine but I reload various 556 bullet types/weights and have never had a malf from an HK magazine. 77gr SMK’s / TMK’s feed flawlessly through HK Magazines into both of my MR556 rifles as well as numerous other 556 rifles.
This is not a BCM rifle, it’s a home built gun for all the people telling him to call BCM. OP, check your carrier key, and check your gas block, this could be a gas issue. I have seen this exact thing happen due to short stroking because the carrier key bolts backed off and it became loose. The BCG does not get enough gas, it short strokes and the bolt lugs catch the side of the cartridge as it comes forward, and wedge the cartridge in at an agle and dent the brass. In my case torquing the carrier key screws back down fixed the issue.
My novekse loved to do this. The barrel extension “corners” on the inside were very sharp. The round pivots on these as it slides forward into the chamber. If they are too sharp, they did into the case and arrest its movement. You will note significant shaved brass in the pre-chamber area. The magazine feed angle can help or exacerbate this issue.
The 3rd Ranger Battalion and Regiment teams nearly lost the 2004 Army Rifle Championships due to the then-new HK mags. A runner from one of their supply rooms saved their butts with a box of GI mags.
Went out yesterday and it functioned perfectly using PMAG’s and Okay mags with Magpul followers.
I tried and could not duplicate the failure with ball ammo and also fed it some HSM 75gr OTM’s, I had one double feed with one of the older Okay magazines.
I have checked the gas key screws and also looked for leakage around the gas block, I don’t think it is short stroking.
What threw me off was the fact that the suspect HK mag worked fine in a couple other rifles but in this one I had several failures, I won’t be using HK mags anymore.
