AR bolt won't lock on last round, why?

[marinville]

Took my rifle to the range yesterday and left frustrated. I had intended to practice mag changes, but the bolt will not lock back on the last round.

Rifle is spikes midlength. Mags were three gen two p-mags. Rifle has seen 600 or so rounds prior to this outing. Mixed tula and wolf gold. Never both in the same outing. Rifle is cleaned properly and lubed well after each range trip. Never had any failures prior to this.

Ammo this time was wolf gold again and also federal champion 223 (I think that's what its called, Walmart 100 round box. Black and maroon box.) Rifle has geiselle (sp?) SSA trigger, spikes st-t2 buffer, and Norgon ambi mag release.

Rifle will lock back when fired from bench with mag as monopod (pushes mag further into well) rifle will lock with single round loaded and fired. Rifle will lock on empty mag w/ charging handle pull. Otherwise, shooting standing offhand rifle will not lock.

I'm open to any helpful suggestions.

FYI: Rifle has seen 200+ rounds (wolf gold) prior, in this same configuration (SSA, Norton, etc) and did not fail to lock then.

[RXM]

Did you try another magazine? How old is the one you're using now?

[marinville]

Did you try another magazine? How old is the one you're using now?

This happened consistently with three different gen 2 p mags which I purchased in July. The mags have seen a total of less than 800 rounds combined.

[scruff]

How well is well-lubed - coated or dripping? I'm betting you just need more oil. Next time it happens, just squirt a good bit of oil into the ejection port onto the closed bolt, let it run down the oil holes and around the bolt carrier, and it will probably run fine.

[sprrdhawk44]

It's either the mags, the boltcatch or the gun is short stroking. Do you have other ARs?

[marinville]

How well is well-lubed - coated or dripping? I'm betting you just need more oil. Next time it happens, just squirt a good bit of oil into the ejection port onto the closed bolt, let it run down the oil holes and around the bolt carrier, and it will probably run fine.

Coated well but not dripping. I just have a hard time believing fouling could have been the culprit when this began after the first three rounds (of federal) fired in this range trip. Previously I have fired up to 200 rounds of tula filth in a range trip with no such failures.

[Fragger]

Isn't the spikes st-t2 buffer a "Heavy" buffer?
Why would you run that on although not mentioned what I'll assume is a Carbine length 16" gun even with a mid length gas system?
Or is this how the upper/lower was built when you purchased it?


I'm thinking gassing and short stroking is your problem.

You say everything is fine when the carrier is locked back by hand.
Also on a bench where less movement of the firearm when fired would be noticed as opposed to offhand shooting.

Try a standard carbine weight buffer would be my suggestion.

[scruff]

Coated well but not dripping. I just have a hard time believing fouling could have been the culprit when this began after the first three rounds (of federal) fired in this range trip. Previously I have fired up to 200 rounds of tula filth in a range trip with no such failures.

Just give it a try. I've been in the same situation, Colt 6920, cleaned and oiled, locking back intermittently after a couple hundred rounds of quality ammo, and a squirt of oil worked wonders.

It has nothing to do with how clean or dirty it is. It just wants to run wet. In fact, it'll run like a top, no matter how dirty it is, as long as the bolt is dripping.

eta: You've got nothing to lose by trying, and if that's the problem, better to find out than to start replacing parts. Usually, the simplest explanation is the correct one.

[marinville]

It's either the mags, the boltcatch or the gun is short stroking. Do you have other ARs?

I don't have other ARs. If the gun is short stroking it wasn't on previous trips because I've never had this issue previously. I've also never had a failure to feed, even with any of the filthy and underpowered steel case shit. Any way to test if the catch is bad?

My confusion comes mostly from the fact that this is a new issue and nothing with the rifle or mags or even ammo has changed since previous flawless range trips.

[marinville]

Isn't the spikes st-t2 buffer a "Heavy" buffer?
Why would you run that on although not mentioned what I'll assume is a Carbine length 16" gun even with a mid length gas system?
Or is this how the upper/lower was built when you purchased it?


I'm thinking gassing and short stroking is your problem.

You say everything is fine when the carrier is locked back by hand.
Also on a bench where less movement of the firearm when fired would be noticed as opposed to offhand shooting.

Try a standard carbine weight buffer would be my suggestion.

The rifle came from the factory with the st-t2 buffer. My understanding is the weight is between an h and an h2.

While I am not entirely opposed to trying a different buffer, I do not understand how the factory buffer could be causing this issue now, when previously it worked flawlessly.