Suppressed only bolt lock malfunction. Ideas? [Resolved]

[kdcgrohl]

I'm having a problem with bolt lock using Pmags while suppressed. I'd like some experienced opinions on the exact cause of the problem. Specifics:
Unmodified KAC SR-15 lower.
H or H2 buffer, doesn't seem to matter.
BCM BCG.
BCM BFH 16" midlength gas barrel, cut down to 14.7" by ADCO.
Permed KAC mount.
FSB is pinned on straight.
NT4 can.

The bolt lock failure is about 80-90%, suppressed ONLY. Unsuppressed, operation is 100% with all magazine / buffer combos.

Magazines:

Revision M Pmags fail to lock back 80-90%
Non-revM Pmags function properly.
USGI(magpul followers) function properly.

Ammo:
Prvi M193, Prvi M855, Fed TBBC.

I realize I could just stick with known working mags, but I would like all my 556 AR pattern guns to share the magazine supply.

Note the follower / feed lip relationship in the pics below.
Also note the divots in the revM followers.

Any ideas, further questions or comments?

*Left to right: Non-revM, revM, revM, USGI w/ Magpul follower.
http://i627.photobucket.com/albums/tt358/kdcgrohl/stuff/2012-03-18_15-28-26_134.jpg
http://i627.photobucket.com/albums/tt358/kdcgrohl/stuff/2012-03-18_15-28-37_926.jpg

[Iraqgunz]

Try an H3 buffer.

[kdcgrohl]

Try an H3 buffer.
Unfortunately, I don't have one handy. I'm going to try a different action spring, and switch lowers and uppers with other known 100% functional rifles and report back tomorrow.

[Clint]

Am I seeing those pics correctly?

It appears as if the followers on the center two mags are sitting low and stopping short of the feed lips.

Something may be up with those.

My rev M do not have any gouges in them.

Followers sitting low would be an obvious source of failure to lock back.

Also check the bolt catch for any signs of wear caused by this.

[kdcgrohl]

They do indeed sit below the feed lips. The first pic displays this best.

Am I seeing those pics correctly?
...<snip>

UPDATE:

This afternoon I tested with four different action springs, SR15, H & H2 buffers. No changes at all.
Then I started looking at the magazines closer. All my black follower pmags(non-revM) work 100%
So I started inspecting all my revM pmags. I have 16, but one of them functions 100%, whereas 15 of them function about 10%...
Dissassembly, and wouldn't ya know it I have two different versions of the revM!
The only difference I could find was the actual follower. The follower in picture left works perfectly, and touches the feed lip.
The one to the right, which I have several of, does not.
http://i627.photobucket.com/albums/tt358/kdcgrohl/stuff/2012-03-19_17-31-50_802.jpg
*It's important to note that this problem is ONLY suppressed. I've had nothing but perfect service from all these magazines unsuppressed.
Yes, Magpul has been contacted and I will update the thread with the resolution.

[kdcgrohl]

The good folks @ Magpul are sending me 15 new followers. By new followers, I mean the exact same followers as pictured right above.
Apparently, the follower left is no longer produced. There was an inline change to facilitate the ACR's linear bolt lock.
I doubt the new followers, which are the same as those I already have, will fix my very specific problem.

In the meantime, I decided to modify the ridge that stops the follower groove in it's upward motion to allow the new revM followers to move all the way up. A quick few single rounds suggest that it works!

I'll give the new followers a try when they get in. If that goes as expected, I'll try to modify the follower or the bodies...

Thoughts?

[Chase45]

Its not really your mags which are the problem.

I had the same problem with my suppressed gun. It functioned fine without the suppressor, but put the suppressor on and I would run into the same problems your are.

Your rifle is overgassed and a better way to fix the problem would be to get a correct buffer and spring.

In my case it took both a H3 buffer and a heavier spring to make the gun work correctly

Actually my rifle is still overgassed as is, but I have not been running into any more problems. I really need to toss this barrel since it has had a bunch of rounds through it

[HELLABEN]

Unfortunately, I don't have one handy. I'm going to try a different action spring, and switch lowers and uppers with other known 100% functional rifles and report back tomorrow.

you have a h and h2, you can take them apart and make a h3.

[kdcgrohl]

you have a h and h2, you can take them apart and make a h3.

Excellent point. Sometimes it's easy to overlook the obvious. Thanks.

[kdcgrohl]

I won't have a chance to really test all this stuff till this weekend but...

Magpul sent me 15 new followers, and it appears they've changed them again.
The new pieces allow the follower to come all the way up and touch the feedlip.
I was bored waiting for the new ones so I also took an end mill bit to all of my "bad" followers.

As you can see, the NEWEST version has a deeper groove up front which allows the follower to travel all the way to the top of the magazine.

Seen below, L-R, NEWEST Magpul follower, modified follower, newest Magpul again, modified again.
http://i627.photobucket.com/albums/tt358/kdcgrohl/stuff/2012-03-28_20-32-26_562.jpg

And here is a before and after of the mods to the "bad" followers:
http://i627.photobucket.com/albums/tt358/kdcgrohl/stuff/2012-03-25_19-19-00_381.jpg

Honestly, I'm not sure that the short channel followers actually caused the problem, or if it was the overgassing, but I think it's interesting that Magpul changed the followers back to allow full extension.

[Clint]

I doubt it's overgassed.

Did Adco open up the gas port when they shortened the BCM barrel?

USGI mags work 100% as do some pmags.

Most likely those suspect mags.

[HELLABEN]

this thread made me start looking at my pmag pile, and i noticed i got 10 of them with the problematic followers, haha

that might explain some of the gremlins i had with my suppressed rifles.

thanks for posting this thread, never would have noticed something like that

[kdcgrohl]

All my Pmag followers now go all the way up, and I have no more bolt lock problem.
The newest followers Magpul sent, and the older units I modified, all work 100%
Thanks for any and all assistance guys.