Shooting one of my 20” uppers and I start to have failure to feed first round. Switch magazines, same issue. Shooting PSA ammo with 77 SMK.  Bolt would jump over loaded rounds or jam on feed ramps.

Gun was clean and well lubed.

I get home from range, and check bcg. Looks ok. Check recoil spring of unknown origin. It measures on the short side of spec. Rifle length buffer weighs 5.2 oz. Change recoil spring out for new Colt rifle length spring.

Back to range today. Exact same ammo, and reloads consisting of RMR 75 jhp, 24 grains Tac in new factory primed Wolf brass. Gun runs 100%  Off the bench, and shooting while standing. Mags were Magpul 10 round mags. They are like new and never been a problem in multiple guns.

I can feel the difference in how smooth the gun chambered the first round and cycled.

Forgive my lack of trouble shooting experience.

Can a defective recoil spring make that much difference? Upper has had hundreds of rounds without a jam prior to this one incident.

It's always a good idea to measure your buffer spring every once in a while. I typically replace buffer springs at the end of the year because they are cheap.

I would think the first round is the hardest to strip off a magazine. Do you lock the bolt back and then insert the magazine to feed it? That will typically seat it correctly and strip the top round in most case.

Just thinking of it, I do believe it may have trouble stripping the first round off if the forward tension isn't as strong.

https://www.ar15.com/forums/AR-15/Troubleshooting/66/

Could be short stroking, too heavy buffer, try lighter buffer.

Check for OOS feed ramps on the barrel extension overhanging into the receiver.  Only takes a c-hair to hang up a round.  I ran into this issue once.  Noticed only with GI mags because they present the round every so slightly lower than Magpul or Lancer.

It was failing to fully chamber the first round and he's running a rifle buffer set up.
OP, sounds like you got it working, but the advice to inspect your feed ramps is a good idea.

Missed first round failure to feed.

It happens

Yes, a worn spring can do this. That's why there is a measuring requirement in the first place per the TM.

Most likely the bolt speed was excessive and the buffer was rebounding off the rear of the receiver extension which *can* recoil the gun hard enough to keep the rear of the ttop round of the magazine tipped rearward and allow the bolt to outrun the magazine as the nylon buffer provides a lot of forward force on the BCG.

This can also exhibit a failure to feed by having the round tip up after the bolt has traveled too far forward to strip the round from the mag and allow the front face of the bolt carrier to do so instead, leading to the top round being partially stripped and jammed up inder the bolt as the tip of the round tries to ride up the feed ramps.

There will be a big assed dent in the brass case as it gets hammered between the barrel extension and bolt lugs.

I bet your brass throw was forward and the gun acted over gassed.

This is why we perform PMCS on the regular.

Attached File

Thanks. I don't have a copy of the 23&P on my phone to drop that little gem in here.