Update:

Well, crap. I took the upper out and put another 120 rounds thru her. Intermittent short stroking across 4 different mags: 2 D&H, 2 PMAG.

I was hoping this was just a break-in period, but now with over 200 rds thru, it may be gas port related. Oh well, just glad that it is a BCM upper given their reputation for good service.

Original post:

Rifle setup:

New BCM 16" upper, carbine gas system.
New Denny's Super Duty BCG AR15-type (also swapped in Noveske M16 BCG for trouble shooting, same result)
Carbine buffer (also swapped lowers with an H2 for trouble shooting, same result)

Ammo used (no reloads):

60 rounds of Centurion 55 gr,
60 rounds of HSM 55 gr,
30 rounds of Hornady 75 gr LE TAP (the hard to find, expensive kind)

Result is the same - failure to lock open/back on an empty magazine and occasional FTFs with the BCG skipping over the round. Consistently failed the short stroke test (never passed, actually on about 10+ tries). I tried swapping lowers, buffers, and BCGs with no difference. This is a new upper from a very reputable dealer/manufacturer. Could this just be initial break-in after this many rounds, or is there a larger issue at this point possibly related to gas port size?

Thoughts? Thanks in advance.

ETA: Its a Denny's Super Duty BCG (ar15 type)

Have you tried different mags.

Don't know the specs on all the ammo you described, but if it's a spectrum of higher to lower power loadings, and with all the other swapping around it does sound like a low gas problem.  Presuming you tried different magazines as the previous poster mentioned, to eliminate something like a bent feed lip causing drag on the BC, and some basic non-firing tests to locate any other anomolies as previously mentioned.

Interesting as most carbine gas 16" barrels range from moderately to severely overgassed.

If you're experienced with an AR you can usually tell a short stroking/undergassed situation right away as you don't feel the secondary recoil impulse when the BC/buffer bottoms out at the back of the buffer tube.

Thanks for the replies.

Yes on multiple magazines. For the short stroke tests I used 2 PMAGs, 3 D&H Indust, 2 Okay.

Yes on cleaning prior. Hit it with the chamber brush, too. I stick to CLP on my ARs and ran it wet. Not soaking, but wet.

I do have a fair amount of time behind the AR15, but did notice one thing somewhat different: every time I fired it, it seemed to sound extra "springy." It sounded like a "sproing" after every shot. Kinda weird. Could that indicate undergassing or overgassing?

I think I need to put some more rounds down range for additional break-in before calling it a gas port issue. I'll post an update later.

Thanks, guys.

If you have a rifle spring in a carbine buffer tube it will bottom out on the spring before the full travel of the buffer+BC is achieved, and will cause short stroking symptoms.  Unless you have the same problem with both of the lowers though that should have ruled that out.

Yeah, positive both lowers are carbine springs. I've never had any issues with either. Good insight, though.