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

[ianb1116]

Might be a gas port/gas block alignment issue. Although with pins, it should be pretty good. Have you tried a different gas block?

[ungawa]

I thought the same thing, that something must be wrong with it. My bolt had a visible film of oil on it, but wasnt "wet". It was a carbine instructor that set me straight - instructors for multi-day classes see more malfunctions than I ever will. Since I've been using more oil, my Colt has been running flawlessly.

He's got nothing to lose by trying it. If you're correct, running it wet will make no difference at all, and he can move on to replacing parts.

If running it wet makes a difference, it's still broken or defective, and I'm still correct.

If you need lube to get enough action to lock a bolt back you have a POS that is barely working. It must have way more getup and go than that to be considered functioning or reliable.

If that's what it does when times are good, what are you going to try next when conditions are bad and it won't strip a round? Play with forward assist?

[Bang]

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.

Reminiscent of what happens when a semi-auto pistol is "limp-wristed".

I would suspect the buffer system. It is reacting differently in that one method of shooting, suggesting a different absorption of kinetic energy or timing. And if copious oiling fixes it, I would think that is an additional indication of buffer tune.

[marinville]

Reminiscent of what happens when a semi-auto pistol is "limp-wristed".

I would suspect the buffer system. It is reacting differently in that one method of shooting, suggesting a different absorption of kinetic energy or timing. And if copious oiling fixes it, I would think that is an additional indication of buffer tune.

I may have explained this poorly. No matter the shooting position, the only way the bolt will lock back is if upward pressure is exerted on the mag or if only a single round is loaded in the mag or if I shoot very slowly.

Even if I fire from the hip it will lock back if there's only.one round in the mag or I shoot slowly or I jam the mag up into the magwell.

[Hawk]

No one mentioned the possibility of a loose gas key. The pattern of the problem fits a gas key that has loosened.

Item 2 is the "gas key". Make sure items 1's are tight and staked to prevent loosening.



Loose gas key:
http://www.youtube.com/watch?v=eMyfPwy0FMg

How to stake a gas key:
http://www.youtube.com/watch?v=2tYWjQPbwS0

[marinville]

The gas key was staked from the factory, but it is one of the things I intend to check when I clean the rifle later this evening.

[dkf]

He can check the gas key. It also would not hurt to pull the upper from the lower, remove the charging handle and make sure the gas key slides over the gas tube easily.(and the BCG does not hang up) Make sure the bolt catch operates freely and nothing is broken or chipped. Won't hurt to have a good once over in case of a fluke part failure.

I'd be picking up more mags too. Some Lancers to try. Only 3 mags doesn't cut it.

[marinville]

He can check the gas key. It also would not hurt to pull the upper from the lower, remove the charging handle and make sure the gas key slides over the gas tube easily.(and the BCG does not hang up) Make sure the bolt catch operates freely and nothing is broken or chipped. Won't hurt to have a good once over in case of a fluke part failure.

I'd be picking up more mags too. Some Lancers to try. Only 3 mags doesn't cut it.

I have twenty gen 2 p-mags but I don't take them all to the range. The rest stay at home, loaded.

I will check everything you suggested when I get the rifle apart.

[marinville]

I just took the rifle down and instected it. The gas key is tight. The gas tube is aligned. I'm leaning towards the lower or the bolt catch being out of spec.

The bolt catch is noticibly loose in the lower. If I push on it with my finger it wiggles. Pushing forward and inserting an empty mag (in the seperated lower) results in engagment of the bolt catch. Not applying pressure on the bolt catch and inserting the same mag results in the bolt catch not being engaged by the follower.

Also, upon further experimentation, inserting an empty mag (in the assembled rifle) and racking the charging handle results in only sporadic lock back.

I still can not explain why the rifle used to lock back reliably or why it was locking back reliably yesterday when only one round was loaded.

I am going to grab a different (fourth) p mag and continue experimenting, but I expect similar results.

I hope its not the lower because I really don't want to bother sending anything back to spikes.

[dkf]

There is usually a good bit of play between the slot in the lower and the bolt catch. I add some stainless shims to all my rifles to remove some of the side to side play. They are a pain in the ass to put in but do the job.

It is possible your catch has wore some from use and now does not want to work right.