11.5" BCM Short stroking

[MistWolf]

Not locking back can also be caused by extremely violent or rapid cycling.

Perhaps, but it would be so violent as to be obvious. I had a barrel that was so grossly over gassed, recoil was sharp and uncomfortable. The carrier struck the hammer so hard the shock stung the trigger finger. But I could not get the carrier to outrun the magazines or the bolt hold open.

[WS6]

Perhaps, but it would be so violent as to be obvious. I had a barrel that was so grossly over gassed, recoil was sharp and uncomfortable. The carrier struck the hammer so hard the shock stung the trigger finger. But I could not get the carrier to outrun the magazines or the bolt hold open.

In OPs case, apparently not, as removing the suppressor made things work properly.

[patrick sweeney]

If it works just fine with everything but the V-max, then don;t use it. If you want to dig in and discover why, go for it, but consider it R&D, not curing a problem. I've tested a metric buttload of firearms over the years, and now and then you just run into a case of "ammo hate." Known, reliable, facotry ammo that works jsut fine in a host of other firearms, for some reason the one being tested just hates. In cases like that, I just don't use that ammo.

[younggrasshopper]

FWIW I started with a standard H buffer in my BCM 11.5 and the brass was ejecting erratically. Switched to an H2 buffer and it resolved the issue.

[0311_jarhead]

I had to install a reduced weight spring in mine to cycle 100%. There are a few makers of reduce weight springs and some are actually aimed at 300 bo specifically. I got mine from Gunsprings.com aka Wolff Springs. http://www.gunsprings.com/index.php?...D=1&dID=79#490 Item 16505 on that link.

[matemike]

If it works just fine with everything but the V-max, then don;t use it. If you want to dig in and discover why, go for it, but consider it R&D, not curing a problem. I've tested a metric buttload of firearms over the years, and now and then you just run into a case of "ammo hate." Known, reliable, facotry ammo that works jsut fine in a host of other firearms, for some reason the one being tested just hates. In cases like that, I just don't use that ammo.

Great idea. It worked with everything suppressed and unsuppressed EXCEPT the V-max suppressed. All I may ever do is check those v-max velocities. See if it was a hot batch or something. Not gonna mod up my reliable gun to make one random round work.

FWIW I started with a standard H buffer in my BCM 11.5 and the brass was ejecting erratically. Switched to an H2 buffer and it resolved the issue.

I have since acquired a barrage of different buffer weights in case I ever run into a frustrating situation like that again. Still running the original H buffer though as it’s worked well thus far.

[jerrysimons]

WS6 hit the nail on the head. You confirmed your gun is in fact OVERGASSED with that particular v-max .223 ammo when you took the M4-2000 off and it cycled. That ammo maybe a hot batch or just have a different pressure curve (is it superfromance?). The symptoms you initially described as well as the 1 o’clock ejection indicated excessive carrier speed.

You didn’t mention what magazines you are using but short strokes don’t cause the bolt to slip off the cartridge rim and drag up the case jamming it into the extension. Excessive carrier velocity outrunning the mag spring causes this malfunction as well as failures to lock back because the bolt gets ahead of the bolt catch before the follower can present the bolt catch up to stop it. A bad magazine can also cause these symptoms in a gun cycling properly.

The AAC M4-2000 is a can known for its high back pressure, BCM’s 11.5” barrels are like 0.076” gas-ports which is not lightly gassed. You are only using an H buffer. I think you were getting by with the other ammo but that v max pushed it over the edge for whatever reason. I bet you a heavier buffer H2 or H3 will solve the problem.

[matemike]

Update:

Made it to range yesterday. Those V max’s are running average of 2870fps; 120 FPS faster than M855, armscorp, and Remington PSP.

I ran many mags loaded with 5 rounds suppressed of all ammo. One particular black p mag had a single failure to feed with the v Max’s and did not hold bolt open on last shot 3 times out of maybe a dozen attempts with v Max’s and didn’t hold bolt open once with the armscorp ammo also out of maybe a dozen.
5 other mags and all other ammo worked flawlessly suppressed. Everything ejecting to 1-2 o’clock.

Unsuppressed everything ran. Even the particular p mag with all ammo. Held open every time too.
For the last check, twice I loaded that black p mag with 5 vmaxs and ran suppressed with a H2 buffer instead of the H1 that I used in all other scenarios. It cycled fine and held open both times suppressed. Then I was out of bullets and it was starting to rain.

One thing I noticed after the 300 rounds yesterday is that the suppressor was getting difficult to take off. I have never cleaned the threads on the suppressor nor the muzzle device. The most I’ve ever cleaned my brakeout is to wipe it with solvent until the carbon quits making my hand black when I touch it. Well, I scrubbed and scraped off a bunch carbon from both the muzzle mount and suppressor threads. Maybe it was locked up so tight that my back pressure was intensified?

I think what I heave learned is that my gun will run them for the most part, but it doesn’t prefer v Max’s suppressed, especially with a H1 buffer or a dirty suppressor mount.

[jackblack73]

It seems like you're reluctant to run the H2, although I'm not sure why. It's not the dirty threads. The bolt is running too fast with the VMax. Just run the H2. I run my 11.5" BCM with standard carbine spring and H2 and it runs all ammo fine, suppressed and unsuppressed. I consider it the standard and run an H2 in all my rifles.

[MontanaMarine]

I'd also take a look at the rim of the fired casings. Look for aggressive extractor marks. Clear indication of over-gassing.

I'm using an H3 buffer and standard spring in my 10.5 and 11.5 guns. They hum right along without issue.