CMMG upper issues

[civilian75]

The only BCG that works came with a milspec M4 built by Charles Daly using mostly Daniel Defense parts (receivers, quad rails, and I guess the BCG). Unfortunately, the BCG has no identifiable (at least to me) markings.

[SpaceWrangler]

It's good you're able to identify a specific make that works for you. If nothing else solves your problem, you might be able to pick up a gently used mil-spec BCG for not much $$$.

There's not much rhyme or reason to AR Bolt Carrier markings.

Some Colts have a 'C' stamped on the left side, but not all.

'J' carriers are a USGI replacement part that are are commonly found on rebuilt USGI M16A1 rifles. They were made by Smith Enterprise for JC manufacturing in the mid 1980's.

'A' carriers are made by Advanced Ordnance Corp, another USGI replacement carrier.

I've seen some DPMS (which I wouldn't recommend using) with a 'D' stamp, and others without. But therein lies the problem... it's nearly impossible to tell a properly made, but unmarked, BCG from a piece of poorly-engineered copycat cheese.

[civilian75]

CMMG BCG finally worked for the first time . Since I only had one round left from the last batch that's all I could test fire. But, the BCG locked open on an empty mag. That was with the unmodified H buffer and milspec buffer spring. Is that a good sign, or not? This is what did it:
* Checked the the gas key as you suggested and lo and behold! Staking was done very poorly. The screws where quite loose. I re-tightened the screws and and re-staked the key. Before and After pics.
* oiled the heck out of the BCG, inside the receiver, and charging handle.

Bottom line, SW, you were right, twice. Tnx. Will need to fire a few more shots before I can confidently give it a clean bill of health. You may expect at least one more update.

I did a couple more things. Took the gas block apart again and re-inspected it much more carefully. There were no signs of leakage. Confirmed gas port holes alignment is fine. I also cleaned the barrel thoroughly and polished it with JB paste. I know that had nothing to do with the BCG performance, but it was just something I had to do. When developing a subsonic load for another rifle/caliber polishing the barrel helped to reduce the velocity spread. And that was with cast bullets. My guess is with jacketed bullets it is much more critical.

Now I need to work on my loads.

[SpaceWrangler]

Capital! I'm glad it was something simple.

[JohnInNH]

Poorly constructed handloads will do that every time!!

You should call customer service and tell them what the problem was. maybe they can issue a vendor corrective action order and have their subcontractor fix the problem, unless of course they assemble their own bolt carrier groups. Regardless they should know about the assembly defect.

[SpaceWrangler]

This is why AR's get a bad reputation among hobby shooters.

Ned Milquetoast buys a hobby-grade AR because his friend that used to be in the military said it was "just as good as a Colt or Noveske, except you're not paying for the name". And then the gun chokes in front of everybody at the range because it wasn't assembled correctly from the factory, and all the good 'ol boys (who know guns so well because they shoot several boxes of ammo every year) say "Yep. Typical AR."

I wish the 'short-bus' AR brands would wise up and make these things the way they were intended. The short-term investment of money and time would come back several-fold over the years as their reputation builds.

Dah. I'll wish us up a cure for cancer while I'm at it.

[civilian75]

It is official, the CMMG is fixed. BCG has full stroked every time after the gas key fix and heavy lubing.

I am not sure the failures had anything to do with bad reloads, though. I am concerned with the wide velocity spread. It is way too early to declare victory but the barrel cleaning, polishing appears to be helping as well. And, it is hard to tell, but maybe even the loose gas key was also messing with my loads, too. But, as I said, it is too early to tell.

[JohnInNH]

There is a problem withslow burning powders in subs or really any load when you do not achieve the optimum chamber pressure needed for best burn. This load may be in the correct pressure but my bet it is in the low range if at all. It is easy to get pressures up with fast powder, but as an example with a 180 gr bullet where 1680 can push it well over 1050 fps, it obviously will be lower chamber pressure for subs.

This is why heavyweight bullets perform better. Your fps ES may just be the nature of the beast. Seating deeper rather than longer may help? But even factury 220 subs had a fairly high ES IIRC.

Your loads had nothing to do with any of this.. It's 100% the rifle and the mfg. Should hear about your problem so they can do something to fix the obvious QC issue. They own it 100%

[civilian75]

John,

I am on the fence about telling CMMG. They screwed me on the warranty thing, why should I help them? If you are a praying person, you are welcome to try swaying me to the other side.

I totally agree with you about the slow burning powders. Quickload is not a bad performance predictor once you learn how to interpret the modeling results. I have learned to prefer powders between 90% and just under 100%. You can sometimes get good performance in the 80s but it is iffy.

I am going to be busy with other stuff the next couple of weeks. But once I am back, I will likely start new threads on my newer findings. For instance, I have noticed that chambering can unseat most bullets anywhere from .002" to .010", and that's even using a Lee Factory crimping tool. Amount of crimp appears to have little effect. New Remington brass has significant amount of neck tension compared to reside brass. But haven't done any testing with resized brass yet. That, combined with a slow burning powder, is certainly not helping mv spread. I wished there was a heavy match grade bullet with canelure for 300 AAC Blackout subsonic work. Maybe if we all Blackout user pester bullet manufacturers...

Now that I have fixed the upper stroking issue, I might try A5744, a slightly faster burning powder, and quite position insensitive for its speed. I am hoping pressures might be high enough at the carbine length gas port to actuate the bolt. Burn rates is slightly higher than 90% per Quickload.

There is yet another issue. While doing the unseating testing I found two rounds would fail to engage the extractor. This happened with two different BCGs. But that did not stop me from closely inspecting the claws. It troubled me that in one case the bolt would not get into battery, rendering the firearm useless. Had to extract with a wooden dowel. Very crude caliper measurements appeared to show that the new & primed Remington brass shoulder dimensions are a bit different to fireformed brass, resized brass with my Hornady dies and Blackout spec. It seems the Remington brass would let the cartridge go deeper into the chamber and out of reach from the extractor claw. It is a very speculative hypothesis at this point but worth exploring. I guess it is one of the hazards of having such a small shoulder.

I wished I could borrow a Go-No-Go gauge set to fully clear the chamber dimensions from fault. This is a call to fellow posters, if you are willing to trust me with your gauges. And of course I'll pay shipping both ways.

[Quiet Riot]

I am on the fence about telling CMMG. They screwed me on the warranty thing, why should I help them? If you are a praying person, you are welcome to try swaying me to the other side.

They read this forum. They already know, I'm sure.