Range Report AR15 - bolt not locking back on last round - why?

[NativeCraft]

So, for my offhand position, I've been shooting 68gr bullets with 24.0 gr. of Re-15 for a while now, with no problems whatsoever with the bolt locking back. Loaded up some 77 SMK's with 23.5 gr. Re-15 to try and on almost every shot (single-load) the bolt failed to lock back indicating and empty magazine/chamber. Any idea why this may be happening?

FWIW, I do position the buttstock really high on my shoulder and I'm thinking that the rifle isn't supported well enough to let the bolt ram all the way back, but why does switching bullets and a small change in powder affect this? Perhaps I'm on the verge of funcionability with the 68gr. bullets and just past it with the 77's???

 

[blksno]

Re: AR15 - bolt not locking back on last round - why?

it's most likely NOT the ammo. it is probably due to your mag or your bolt catch.

(oh, and yes, it could be that your bolt is coming back far enough to kick a shell, but not far enough to clear the bolt catch. have someone watch you shoot, i had this problem when my bolt key got loose and was losing gas)

Put the empty mag back in your set up...and guide the charge handle back nice and slow while you look inside. you should be able to see the follower push the bolt catch up to engage the bolt once it clears the follower.

sometimes the follower doesnt engage the bolt catch hard enough. sometimes you need a better spring in your mag. sometimes your mag is riding too low in the mag well. could need to lube your bolt catch (but i'd doubt it)

just do it slow and take a peek

 

[NativeCraft]

Re: AR15 - bolt not locking back on last round - why?

I'll take a look at it, c_bass - but if it was a problem with the magazine/bolt catch, why would it work just fine with the 68gr. bullets?

Oh, and what's a "bolt key"?

 

[Powder_Burns]

Re: AR15 - bolt not locking back on last round - why?

Pretty convinced its your 77gr handload. Not getting enough gas to lock the bolt back. If your 68gr loads will lock the bolt back then you can rule out the bolt catch, gas impingement system, or magazine. Work up some hotter loads, switch powders, etc. until it cycles fine. Some powders are less than ideal for the 5.56mm case and only start to cycle proper when you are nearing max charges, first powder that come to mind is H4198.

My lyman book shows the 77gr, at 2.260" MAX charge 24.5gr with Re-15. Starting load is 22gr. You are right in the middle of that spread with 23.5gr so you have a little more room to go...of course watching for the usual pressure signs etc. Might want to give BL-C2 or H335 a try as well..

 

[NativeCraft]

Re: AR15 - bolt not locking back on last round - why?

Thanks, Powder.

 

[blksno]

Re: AR15 - bolt not locking back on last round - why?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: NativeCraft</div><div class="ubbcode-body">I'll take a look at it, c_bass - but if it was a problem with the magazine/bolt catch, why would it work just fine with the 68gr. bullets?

Oh, and what's a "bolt key"? </div></div>

i should have said "bolt carrier key". my bad.
the lil tube shape deal on top of your carrier that the gas tube fits into. mine somehow got loose one day and I was losing gas preventing my bolt from going far enough.

I was just pointing out hardware issues that could cause the problem. Powder is probably right with your charge weight being the culprit. i've just never had my loads be the issue.

 

[NativeCraft]

Re: AR15 - bolt not locking back on last round - why?

Thanks, c_bass. I had no idea what that piece was called. I can see where it being even slightly loose could cause a problem.

 

[Victor Co Heather]

Re: AR15 - bolt not locking back on last round - w

Short Stroking is a gas system problem. It's generally caused by one of these things:

1. under powered ammo
2. too heavy of a buffer (like a H2)
3. loose carrier key

 

[johngfoster]

Re: AR15 - bolt not locking back on last round - w

Had it happen to me with my Bushmaster. Would cycle just fine with XM193, but when I shot Black Hills 40gr V-MAX, it would not cycle far enough to pick up the next round and I'd have to rack the charging handle each time. Turned out the gas key on the BCG was loose.

 

[rmfield]

Re: AR15 - bolt not locking back on last round - w

Lots of different things can cause short stroking, but the most frequent cause I have seen is worn out gas rings. If you have lots of rounds through your AR and have never replaced the gas rings this would be worth checking out. If the bolt flops in and out without any friction, this is a good indicator that the rings need to be replaced. The extended bolt should support the weight of the carrier if you stand it up on the bolt face. Otherwise, check the entire gas system for blockages.

 

[Zebra644]

Re: AR15 - bolt not locking back on last round - w

An idea is also to use some standard 55 grain Winchester or Remington UMC Federal or Military grade 5.56. Use 1 mag with that kind of ammo, see if you have any malfunctions. Use the same mag and try your reloads. If the factory stuff works and the reloads dont, well I would lean towards the reloads being the issue. If the weapon still short strokes, with both the reloads and the factory stuff then check your bolt catch, then check the bolt carrier key for tightness, and check the 3 gas rings on the rear of your bolt and make sure that the seperations on the rings dont line up. I have heard that if you line them up that gas will escape through that opening and it may casue it to short stroke. Also check your gas tube, use a long pipe cleaner and clean it out.

 

[smokshwn]

Re: AR15 - bolt not locking back on last round - w

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: M.45</div><div class="ubbcode-body">Short Stroking is a gas system problem. It's generally caused by one of these things:

1. under powered ammo
2. too heavy of a buffer (like a H2)
3. loose carrier key



</div></div>

It can also be caused by the gas block being slightly out of alignment and partially blocking the gas port.

When checking the gas key also check to see that the end of the gas tub has a slight resistance going into the gas key. If not excessive gas can be lost slowing down the carrier.