6920 lower, BCM 16" LW middy upper w/Mod 0 comp, carbine buffer, Wolff std buffer spring. Completely reliable w/everything from PMC Bronze to Wolf Gold using the BCM bcg.
Tried a new bcg, initially unreliable feeding/lockback w/Wolf Gold. Changed gas rings, now 100% reliable w/Wolf Gold, functions reliably w/PMC Bronze but won’t lock back reliably.
Picked up a 2nd bcg from the same mfgr, same initial problem as the first. Changed gas rings, no improvement (I’ve emailed the mfgr).
To troubleshoot I switched bolts between the two bcg’s hoping to isolate the cause of the problem.
Older new carrier w/newest bolt functioned reliably w/Wolf Gold but wouldn’t lock back reliably. PMC Bronze = FTF’s.
Newer carrier w/older bolt functioned reliably w/Wolf Gold but wouldn’t lock back reliably. PMC Bronze = FTF’s.
Here’s where it gets strange:
Swapped bolts back to their original carriers and now the previously reliable older new bcg still functions reliably w/both Wolf Gold & PMC Bronze but won’t lock back reliably w/Wolf Gold. This was tested w/9 different previously reliable Pmags, BHO shows no wear and moves freely.
So what’s going on here? Are my bcg’s cycling too fast for the BHO to function? Do I need a heavier buffer? Newest carrier eating gas rings?
All constructive input welcome. Thx!..
Tomac
Difficult to say, subjectively. Ejection pattern’s the same, but cycling too fast might explain the lack of lock back w/both weak & full-power ammo. However, the BCM bcg was 100% w/everything using the carbine buffer.
Tomac
ETA: BCM bcg was also good w/an H3 but new bcg exhibited same behavior w/the H3 as w/the carbine buffer.
Already looking into returning them, but would sure like to know what the specific problem is, the symptoms don’t make sense to me (ie: reliably functions but won’t lock back w/either weak or full-power ammo). Shucks and other comments…
Tomac
Do the mags you were using function reliably in other rifles? Have you tried this upper on a lower that is known to work reliably? How about some different ammo other than the two you mentioned; sometimes, one brand is just too weak to reliably lock back the bolt in some set-ups. Lube situation? Was this upper well-lubed and with what?
Used 9 different Pmags that were 100% in the past. Watching to be sure it wasn’t one or more mags causing the problem, I noticed that the infrequent lock-backs w/Wolf Gold (1/3-2/3 of the time) were random w/regard to the mag used.
These uppers (I have 2 identical setups that perform identically) were 100% w/their 6920 lowers until I replaced the bcg’s (I tested both uppers to eliminate a single upper as being the problem).
Everything was well-lubed w/Mobil 1 (been using it for some time w/o incident).
I shoot Wolf Gold as my practice ammo and use PMC Bronze to test the reliability of a new setup.
Before changing the bcg, both uppers were 100% (including lock-back) with both those loads.
If undergassed, I should be getting short-strokes w/PMC if the bolt isn’t locking back on the more powerful Wolf Gold, not reliable functioning (and that is what’s really bugging me, the symptoms don’t make sense).
Tomac
PMC Bronze .223 (55gr). I don’t have digital calipers, only an analog set from reloading (accurate enough?). Where specifically would you have me measure? Thx!..
Tomac
The part I missed was that these were 100% reliable with same loads, uppers, etc UNTIL new BCGs. So, yes, has to be something about the new BCGs that changed the equation.
Have you tried one of these carriers with a BCM bolt of known reliability? I would try to isolate exactly WHAT it is about the new BCGs that’s the problem–can’t be gas rings since you changed those… Hmm.
Both uppers are still 100% w/BCM bcg’s so the problem has to be w/the new bcg’s (duh). What’s bothering me is that the symptoms don’t make sense, especially those after the recent bolt switch in an attempt to isolate the problem in the new bcg’s.
Tomac
