Compare the center of the barrel feed ramps to the center of the front take down lug.

Could be that when you were tightening the barrel nut (read should have been 35fl of torque, then no more than 80ftlbs to get the barrel nut gas tube passage to align with the upper receiver gas tube channel), the barrel could have slipped, with the barrel extension lugs no longer correctly aligned to the bolt lugs.


Also on the subject of the gas tube channel, with the carrier alone (bolt pulled off) dry fit it only the carrier into the upper receiver to confirm that the gas tube is correctly indexed with the  carrier key, and that the gas tube is free moving in the upper receiver/barrel nut channels for it (see part one about making sure that the upper receiver and barrel nut channel as in line.

No you have a steel pin in a aluminum slot, and can either bend the extension pin, or dent the side of the barrel socket slot to end up with the barrel slipped in the upper barrel socket.

So again, with the upper shotgunned open, just compare the center of the feed ramps with the  center of the front take down lug just below i the center of the ramps.

What the hell happened to the face of the barrel extension????
Looks like someone was pounding at it with a 6lb sledge hammer.

Next is the signs of low bullet feeds striking the front of the upper receiver below the ramps.

And yes, the barrel extension looks rotated in the upper receiver, but looks like the extension pin was bent pulling the barrel off, and not slipped the other was as the barrel nut was being re-tightened.

Could just be the angle of the photo,

But I use the center of the gas tube channel, and the center of the front take down lug.


As for the real test, pull the upper receiver off the lower, then with it upside down, insert the B/C in and see if the center of the bolt lugs are indexing center wise with the receiver extension lugs.

Next to weed out a chamber problem alone (such as not getting it correctly clean, or a problem with reloaded ammo), without the B/C, muzzle straight down, drop a round into the chamber, use a wood dowel, tap the back of the case to make sure that the shoulders of the round are fully mated against the shoulder in the chamber, then turn the muzzle straight up. The live round should fall right out.


If a factory loaded  live round does not fall out of the chamber (and you have been using a chamber brush to clean the chamber with CLP, then using a sharpie pen, and smoke the bullet of the round.  Now do the test again, and this time after you use a cleaning rod to tap out the live round from the chamber, look at the bullet for any signs(scrap marks on the smoked section of the bullet) of embedding into the rifling at loading.

If the problem is with reloaded ammo, then first use an ammo test gauge.  This will tell you if the cases where not correctly trimmed, if you are long loading too long, or even if you over crimped the bullet, ending up with the shoulders of the case slightly bulged.

With reloads, things to check.


That you have set the sizer to kiss the shell holder under  actual ram tension of sizing a case.  If you set the sizer up without this increased tension, then bank when you went to a sizer a case, there was a huge gap between the sizer and holder, meaning that the case was not fully sized.

You have to trim the cases, or they are going to be too long after even a single to few firings.  I have listed this after sizing, since the case needed to be sized first, then trimmed.

When seating a bullet, a little crimp pressure goes a long way!!!!!  If you try to use as much crimping pressure as say a hand gun round, your going to budge the shoulder of the case every time.  Here, a round test gauge comes in handy to check some of the loaded rounds from time to time.  And unless you are using a Lee FLD crimper, then trimming the all the case to uniform is required.

Lastly, if you are loading to come out of the mag, should not be a problem, but when you start to long load to sled rounds, make sure that you are not hanging the bullet so far out of the case that you start rifle embedding on the charge.