FTF with new Centurion upper

[maximus83]

Was the BCM BCG just manually cycling or a test firing?

Is the friction present with an empty chamber?

Just manual, haven't been back out with it yet. Yes friction was there empty.

[markm]

Highly indicative of short stroking. I had this many years ago with a carrier key leak due to one of the key fasteners breaking. You couldn't tell at first glance, but there was just enough gas leakage to do exactly what you're seeing.

Also have had a few worn gas tubes were they leaked gas at the key. Short of a freak error in the port size, sounds like a gas leak somewhere in the chain.

[themonk]

I have had this with 2 BCM carriers. BCM replaced both but I would try with a known good carrier and see what happens.

[maximus83]

Got to take it out for a 2nd try today, tried a few things. I think it's short-stroking, because of the BCG.

1. Fire one round only from mag. Result: never would lock back with the ammo types I tried earlier, or any .223. So with that and the original behavior, seems certain it is short stroking.
2. Swapped out the BCG for a known good LMT BCG. Result: issue solved. I couldn't get it to fail with any type of ammo.

On a side note, once the above was done, found the upper to be extremely accurate, it likes BH 77gr 5.56. Few groups, 1 and 2 are 5rd, 3 is 10rd.

grp1.jpg
grp2.jpg
grp3.jpg

Here's the thing: inspecting the Centurion brand new BCG that has maybe 120rd on it, it looks perfectly fine. Clean, plenty of lube, can't see anything that looks like an issue around the bolt, lugs, gas key, staking....it looks like a very well made BCG in good shape.

I'm starting to wonder based on what Centurion told me about their design (they make their uppers for milspec carbine RE only, running full-power loads only). Is it possible they're doing something as well with their BCG that is BY DESIGN that would be causing an issue when running on my A5 lower? Because they weren't surprised at all when I told them of this issue, they had heard of it before. Still, they sell BCG's as a standalone item and I'd think they'd have to make them to work with a broad range of setups.

I'll talk to them Monday and see if I can get a return on the BCG. But curious now if this will turn out to be a "by design" issue, and I'll keep the upper but simply have to run it with a different BCG.

[1168]

Assuming it doesn’t hurt the warranty, try tightening the gas key screws. Don’t use a lot of torque…you’re looking for a sheared head or an outright loose screw. Sometimes they’re not obvious by trying to wiggle the key, but still leak enough to short-stroke. I wouldn’t really expect this from Centurion, but I’ve seen it quite a bit from other makes. Shit happens.

[georgeib]

In addition to what @1168 said, from watching a bunch of SOTAR videos, I would say your BCG is gas inefficient in some way. I would check the gas rings to start, and maybe try a different bolt and a different carrier with that bolt so as to see if it's the carrier or the bolt.

[maximus83]

Just tried checking the gas key and screws for tightness--they were completely tight and screws well staked. The gas rings looked normal too.

Anything particular to look for on the gas rings?

[cl2]

Just tried checking the gas key and screws for tightness--they were completely tight and screws well staked. The gas rings looked normal too.

Anything particular to look for on the gas rings?

You can assemble the bcg, extend the bolt, and set the whole bcg on a table resting on the face of the bolt (back end of the bcg in the air). The bolt should stay extended in the carrier. If the carrier slides down your rings may be bad.


Sent from my iPhone using Tapatalk

[vicious_cb]

Just got a Centurion CM4 LW 16" midlength upper, first time out had repeated failures to feed. It would fire a round, maybe two or three, then fail to feed. By fail to feed, I mean failing to strip next round out of mag.

Details:
* Gas port: 0.075"
* BCG: new Centurion one
* Mags: Rotated among three newer Gen3 Pmags
* Ammo: BH 77 OTM, Prvi Partisan 5.56 M855
* Lower: BCM MK2 (A5) with their T0 buffer (3.8oz) and milspec rifle action spring

Mags and ammo should be ok, full power loads and fired just last weekend through two other rifles.

Contacted Centurion, they said their built uppers and rifles are not designed for A5 system, they build theirs around the assumption of all milspec. So I agree with them, it's likely not an issue with the upper as built, just that I need to tune my setup to work with this upper.

Assuming best place to start is try a couple of different buffer weights. Ordered BCM buffers T1 (4.7oz) and T2 (5.6oz).

Does that sound right? Anything else I should look at?

As someone who has been running the A5 system exclusively for well over a decade I can tell you that for some odd reason guns get more reliable with the heavier buffer weights on the A5, even small gas port barrels. I did some testing with some small gas port midlengths: a 0.072" 16" mid and a 0.075" 14.5 and found that my ejection pattern got more erratic and weaker as I went down in buffer weight and when I went to the heavier H3 or H4 the ejection pattern and recoil impulse seemed more consistent and finally settled on running H2 or H3 as my preferred weight for almost all guns.

I know its counter intuitive because if you think you are undergassed you should be going with less buffer weight not more but with the A5 I have found that running something like the H2 weight tends to fix problems that only appear with the lighter buffers. I don't know what the principals are behind it(maybe some engi like Lysander can explain) maybe some momentum based or lock time thing but I have found the H0 buffer weight actually cause MORE problems in ALL guns regardless of gas port sizes.

Try it with an H2 and your Centurion bolt carrier, you might be surprised with what you find. I pushed my small gas port barrels all the way up to H4 weight with zero issues with the caveat only some increased muzzle dip but the guns actually felt that they ran smoother. Just when you think you understand the AR you can still run into voodoo magic things that dont make any sense.

[maximus83]

You can assemble the bcg, extend the bolt, and set the whole bcg on a table resting on the face of the bolt (back end of the bcg in the air). The bolt should stay extended in the carrier. If the carrier slides down your rings may be bad.


Sent from my iPhone using Tapatalk

Tried that, bolt stays extended.