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

[sprrdhawk44]

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.

I missed this the first time around. It's defintely the mags or bolt catch. Get a couple new Pmags and see what happens. If that doesn't work, it's the bolt catch. If it's the bolt catch, have spikes take care of it.

[marinville]

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.

You're right that more lube is probably the simplest remedy to attempt, and I will try it. The problem is I have one more range trip to possibly figure this out before I will be shooting a competition event where speedy reloads will be important, so I'm trying to explore all possible causes before returning to the range, and I just can't make sense of what is causing this. My issue with the lube angle is that the gun previously functioned properly through hundreds of rounds of dirty weak steel ammo and then failed after three rounds of fed this time, all with the same amount of lube.

Nonetheless, I will add more lube next time, and I appreciate the input.

[Fragger]

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.

That is a good point but what if your rifle was cycling just on the cusp of 100% when new out of the box?
You are using the same mags right?
I don't see anything changing there.

Ammo?
A few different kinds like you have used before without a hitch.

What about gassing and the effect on cycling?
Well have you removed the gas tube and gas block and cleaned and scraped out the carbon and barrel vent hole?

Of course not and I am not suggesting to do that as routine maintenance. I don't think anyone would.

The fact the gun works fine from a more stable platform such as the bench vs offhand shooting being problematic a small percentage of the time certainly screams gassing/buffer weight to me.

I've built up a few short barreled pistols in the AR platform and always had to play with buffer weights to get cycling for ALL types of ammo at 100%.
What you are describing is exactly what I have gone through and I'm just trying to pass on my experiences from those situations.

Adding another thought/question:

Is your gas block held on with set screws or is it (taper) pinned in place??????????????

[marinville]

That is a good point but what if your rifle was cycling just on the cusp of 100% when new out of the box?
You are using the same mags right?
I don't see anything changing there.

Ammo?
A few different kinds like you have used before without a hitch.

What about gassing and the effect on cycling?
Well have you removed the gas tube and gas block and cleaned and scraped out the carbon and barrel vent hole?

Of course not and I am not suggesting to do that as routine maintenance. I don't think anyone would.

The fact the gun works fine from a more stable platform such as the bench vs offhand shooting being problematic a small percentage of the time certainly screams gassing/buffer weight to me.

I've built up a few short barreled pistols in the AR platform and always had to play with buffer weights to get cycling for ALL types of ammo at 100%.
What you are describing is exactly what I have gone through and I'm just trying to pass on my experiences from those situations.

Adding another thought/question:

Is your gas block held on with set screws or is it (taper) pinned in place??????????????

Mags are the same. Its possible that the rifle was just barely cycling enough to operate properly before, but what makes me doubt that is that I've run a bunch of tula which i believe is pretty weak stuff, and it functioned. And now im having these failures to lock back using wolf gold which I've seen chronos for and its supposedly loaded to almost 5.56 speed.

Gas block is pinned, and I've never removed it or removed the gas tube.

As far as the rifle functioning from the bench, I believe that is a result of the mag being pushed up farther into the magwell (monopoding) rather than a stability issue, because the rifle would lock back when fired from the hip (no shoulder behind the stock) provided that I only loaded one round in the magazine.

I don't mind trying a different weight buffer in principle, I just don't want to start changing things without understanding the problem, and right now I don't understand the problem. Still, I appreciate the advice

[ungawa]

Wakefield knows a gunsmith out your way that can probably take a look at it

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.

If you rifle won't lock back on the last round it is defective or broken, lube or no lube. I can't believe the garbage some people are willing to put up with and even potentially bet their lives on.

Check the gas and buffer systems. OP rifle history suggests likely not magwell, bolt stop, or mag, but check those as well.

[dkf]

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.

I don't see your buffer as an issue. I run an ST-2 buffer on my BCM mid and Spikes mid all with M16 BCGs. Both guns run everything I have run in them, 5.56, .223 brass and steel cased 55gr-75gr. (including cast loads below book starting load) The only time it will not lock back is with a couple GI mags I have. Some of the smaller Pmags are known to have this issue too. I like and use Lancer mags a lot you may want to pick up a couple to try.

You could take the chamber brush and some solvent to the chamber and clean it up good just to make sure you don't have a sticky chamber. Then take it out again.

[Fragger]

dkf brings up another thought.
Are you running a FA or SA type carrier?

I do think it is wise to make sure when switching from steel case to brass and back again that your chamber is squeaky clean to rule that out of the equation.

Also what is your rifle spec chambered in ...
5.56 - .223 or the Bill Wylde configuration?

It can be pretty tough to point to any single thing and say THIS will be your fix because my rifle did this or his rifle works with that set-up.

All we can do is make suggestions based on our past interactions and performances.

And saying well if it worked before why isn't working now sometimes isn't a given nor prudent. If that were the case I might still be driving my 1964 Corvair I bought as my first car in 1972. Well maybe not.

[scruff]

If you rifle won't lock back on the last round it is defective or broken, lube or no lube. I can't believe the garbage some people are willing to put up with and even potentially bet their lives on.

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.

[marinville]

Fa carrier. 5.56 chamber. Chamber was cleaned after shooting steel. Will try to clean more thoroughly this time.

[Fragger]

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.

Agreed. Nothing to loose and may indeed work.

But then I asked myself is/are there any circumstances where I may not be able to run the rifle in this wet condition and I came up with a few of them.

So my thought pattern came back to why not get it to function 100% with a mixed variety of ammo under most conditions that may be encountered by not using a fix that in turn under certain circumstances could possibly present a failure.

LoL this could go on all day long and no one is right or wrong but rather each opinion is just that.

Good luck on the fix.
I'll be interested to see what the OP does.