Bolt not locking back on two different weapons at the same time? [Archive]

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Eurodriver

01-02-12, 19:44

Kinda puzzled at this, its not really a big deal but its never happened before and I'm trying to figure out why.

Today I was running my two SBRs hard. One is a complete BCM 12.5" SS and the other is a 10.3" with a small gas port.

After about 300 rounds each, I grabbed the 10.3" w/ M4-2000. It cycled fine, but failed to lock back after the last round. This was with a PMAG.

Not really testing anything at this point, I just grabbed the 12.5" and put the suppressor on after a few minutes of cooling and fired another mag (USGI). It too failed to lock back!

I fired another 200 rounds through each weapon with no issues, but I did have a failure to lock back on the 10.3" suppressed a few weeks ago with a brand new USGI mag and magpul follower.

It seems the only thing they all have in common is the ammunition as its happened with three different magazines in two different weapons. So far, it has not happened unsuppressed.

Ammunition is Federal XM193.

I could understand if they were not locking back UNsuppressed, but the suppressor increases gas pressure. Shouldn't it have no issues getting all the way back? And with two different weapons? Both lowers have H-buffers and USGI tubes and springs.

As I said, its not really a huge issue but I'd like to at least have an idea why this is occurring.

PRGGodfather

01-02-12, 20:08

Suppressors and SBRs change things significantly.

You may wish to consider going to H3 buffers, so your bolts will lock back. We experienced the same thing with some older full-auto carbines that ran great with standard carbine buffers until we suppressed them. Going to the H3 buffers fixed the problem.

Good luck, and Happy New Year!

markm

01-02-12, 20:23

Agreed. Heavier buffer likely needed to slow that turbo bolt speed down a bit.

Magic_Salad0892

01-02-12, 20:52

10.3'' with .058 port.

If you're running a standard carbine receiver extension, you still want an H2. However, without an LMT Enhanced carrier, you might not be able to cycle.

Iraqgunz

01-02-12, 22:13

I had the same issue in my stock 11.5" suppressed upper until I went to an H3. I think your solution is a heavier buffer (which I believe was mentioned previously) when you asked about build advice.

Clint

01-02-12, 23:34

Is the cycle rate soo high that the bolt is outrunning the catch without the heavier buffer?

RetreatHell

01-03-12, 00:27

skyugo

01-03-12, 00:29

Okay I'm confused here. My brain is in need of knowledge, please feed it...

If the bolt doesn't lock back after last round fired, I though it was because it was short-stroking a very small amount. So this is not the case? Or would that only be the case if it were not locking back when unsuppressed, or with a 14.5-16" mid-length or something?

I guess I'm mainly not understanding how a heavier buffer would help it lock back on an empty mag....?

And how does the LMT enhanced carrier resolve the issue? I actually have one and tried using it with my DD Mk18 (with the DD bolt) and the Vltor A5 RE (with both the standard and one of the heavier buffers) and had issues with it not locking back. Got fed up with it and took my A5 REs off both lowers they were on, but have yet to further test it out since (I was wanting to run it in a class back in Nov and got pissed when it had the lock-back failures and didn't trust its reliability, so it's been sitting by itself in the corner ever since).

My head hurts:confused:

there's a sweet spot. if the bolt goes too fast it'll bounce off the back of the extension and outrun the catch, if it goes too slow it'll never reach the catch. It seems that too slow a bolt speed is rare in carbines. lots of guys running H and heavier buffers.

Eurodriver

01-03-12, 04:48

Okay I'm confused here. My brain is in need of knowledge, please feed it...

If the bolt doesn't lock back after last round fired, I though it was because it was short-stroking a very small amount. So this is not the case? Or would that only be the case if it were not locking back when unsuppressed, or with a 14.5-16" mid-length or something?

I guess I'm mainly not understanding how a heavier buffer would help it lock back on an empty mag....?

And how does the LMT enhanced carrier resolve the issue? I actually have one and tried using it with my DD Mk18 (with the DD bolt) and the Vltor A5 RE (with both the standard and one of the heavier buffers) and had issues with it not locking back. Got fed up with it and took my A5 REs off both lowers they were on, but have yet to further test it out since (I was wanting to run it in a class back in Nov and got pissed when it had the lock-back failures and didn't trust its reliability, so it's been sitting by itself in the corner ever since).

My head hurts:confused:

Thats exactly what I thought too. Bolt not locking back = not enough gas.

Apparently it can mean the complete opposite as well.

As far as the LMT enhanced carrier goes, I have no idea. Its voodoo science ;)

I had the same issue in my stock 11.5" suppressed upper until I went to an H3. I think your solution is a heavier buffer (which I believe was mentioned previously) when you asked about build advice.

I have every intention of going to H3 buffers, I just wanted to make sure (on the 12.5") that I wouldnt have issues with it not cycling unsuppressed with such a heavy buffer. I wanted to confirm before I jumped on it.

Also, I think I will spring for at least one A5 RE. Ill try that lower with both uppers and see what happens.

Eurodriver

01-03-12, 04:57

Guys,

Before I spring for an A5. I'm looking at the RE and the spring+buffer combo that BCM sells. Is this the correct buffer I need? Are there different weights?

I don't want to order an A5 kit and still have to end up buying a new buffer.

markm

01-03-12, 07:01

Thats exactly what I thought too. Bolt not locking back = not enough gas.

Apparently it can mean the complete opposite as well.

It then falls back on the shooter's experience. You can tell when your gun is running short strokey.... vs. running hard. Observing the brass ejection "umph" will give you an idea of which problem you're having.

markm

01-03-12, 07:03

I don't want to order an A5 kit and still have to end up buying a new buffer.

The standard buffer should be just fine for almost every application. 100% supressed in a gassy gun might benefit from the next heaviest Buffer.

skullworks

01-03-12, 07:13

Thats exactly what I thought too. Bolt not locking back = not enough gas.

Apparently it can mean the complete opposite as well.

As far as the LMT enhanced carrier goes, I have no idea. Its voodoo science ;)
Try diagnosing a rifle or carbine with adjustable gas block and you're in for an experience... :bad:

markm

01-03-12, 07:14

Try diagnosing a rifle or carbine with adjustable gas block and you're in for an experience... :bad:

You'll NEVER see me with one of those goofball devices on my gun.

WOW! :rolleyes:

I can only imagine the lifesuck involved in that....

skullworks

01-03-12, 07:19

Eurodriver

01-03-12, 07:25

K, I just ordered an A5 assembly and have an H2 already.

Because Pappabear and IG and Mark and many others have said that the 12.5" is amazing with the A5, I'll try that first and the H2 in the 10.3"

I'm sure that will fix my problem, but if running the A5 on the 10.3" is reliable I'll pick up another one.

Damn you guys and all your A5 talk.

markm

01-03-12, 07:26

Yeah, I was told the other day that I was somewhat of a "milspec-taliban" (yeah, that is the exact quote) for not wanting to deviate from Stoner's design of the gassystem (ie I do not like nor trust adjustable gas systems in an AR).

If something better ever comes up, I'd try it.

But nothing has surfaced yet that doesn't introduce many more problems in hopes of fixing IMAGINARY problems.

skullworks

01-03-12, 07:41

Amen! :agree:

Clint

01-03-12, 12:45

Is the cycle rate soo high that the bolt is outrunning the catch without the heavier buffer?

Thinking about it more, in a moment of clarity, there is another possibility that is probably more likely.

That the failure to lock back is due to short stroking, but the short stroking is due to pre-mature extraction.

The concept of early / late extraction is all in reference to when the case is "done".

Done being pressure dropped to a reasonable level, case cooled and contracted off the chamber walls.

This all takes time.

The longer you wait, the easier it is.

A case that is still pressurized is wedged/stuck to the chamber walls and will take much more work/energy to remove than a "done" case.

The extra work required to remove the case takes away from the work available to compress the spring and cycle the action.

Suppressors give you a double whammy in this regard by extending the pressure curve.

First, the BCG reaches higher speeds from the longer pressure curve and unlocks earlier, and Second, the case takes longer to depressurize and be ready for extraction.

Hot chambers also increase the time it takes for a case to be done.

Smaller gas ports, longer gas systems, more carrier mass and longer cam tracks delay extraction and help compensate for this.

Therein lies the benefit of features like small/mil-spec gas ports, mid length carbines, H2/A5 buffers and LMT E carriers.

As a real world example, case heads of IMI M193 shot side by side from my 16" middy AR and a SCAR16 showed remarkable differences in burrs and extractor marks.

The brass fired from the AR showed very mild swipes and occasional burrs, while the brass fired from the SCAR was remarkably smooth, showing virtually no marks at all.

Any potential chamber differences aside, I believe the biggest factor is the super long cam track of the SCAR that delays unlocking and extraction significantly longer than most ARs.

Steel case ammo takes longer to be "done" and I believe that is why many ARs, especially over gassed ones, have issues with steel.

Tweak

01-03-12, 21:08

I've seen that too but occasional Failures to Extract or Eject are often present in addition to distortion of the extraction rim.

BufordTJustice

02-03-12, 22:57

Okay I'm confused here. My brain is in need of knowledge, please feed it...

If the bolt doesn't lock back after last round fired, I though it was because it was short-stroking a very small amount. So this is not the case? Or would that only be the case if it were not locking back when unsuppressed, or with a 14.5-16" mid-length or something?

I guess I'm mainly not understanding how a heavier buffer would help it lock back on an empty mag....?

And how does the LMT enhanced carrier resolve the issue? I actually have one and tried using it with my DD Mk18 (with the DD bolt) and the Vltor A5 RE (with both the standard and one of the heavier buffers) and had issues with it not locking back. Got fed up with it and took my A5 REs off both lowers they were on, but have yet to further test it out since (I was wanting to run it in a class back in Nov and got pissed when it had the lock-back failures and didn't trust its reliability, so it's been sitting by itself in the corner ever since).

My head hurts:confused:

Retreat,

As I understand it, a heavier buffer usually causes the BCG to remain 'bottomed' at the rear of the RE for a fraction of a second longer than a lighter buffer. And this is in addition to delaying unlocking and slowing the overall speed-of-cycling.

I suspect that the LMT E-carrier coupled with the A5 system may have been too much of a good thing. I would try it with the A5 system and a std FA carrier and see how that works out.

Thomas M-4

06-14-12, 21:28

Retreat,

As I understand it, a heavier buffer usually causes the BCG to remain 'bottomed' at the rear of the RE for a fraction of a second longer than a lighter buffer. And this is in addition to delaying unlocking and slowing the overall speed-of-cycling.

I suspect that the LMT E-carrier coupled with the A5 system may have been too much of a good thing. I would try it with the A5 system and a std FA carrier and see how that works out.

Mine seems to be working fine but my configuration is with a 14.5'' LMT upper and the enhanced BCG and the vltor a5 buffer system. I would be careful on the selection of the enhanced carrier there are only certain configuration that i would try it in. When it works its great but if it doesnt it will be under gassed.

Lockup1109

06-14-12, 23:05

Also try replacing the bolt catch springs in both lowers along with a heavier buffer and a chrome silicon buffer spring.

Thomas M-4

06-14-12, 23:32

Just my perspective but ill try to explain it . You have to take every thing in pespective you have a gas port that slowly over time changes slightly ,gas tube and gas rings that change to wear & parts that break in [OH shit I done said it] that all have to be taken in you equasion . Also you ammo choices could and can play a big part.