Have a new 16" carbine HBAR upper. Kept the used BCM BCG I had & charging handle. Mounted it to my lower(LMT) & zeroed it & shot 300 rounds Saturday with no problems. I took it home, cleaned & lubed it & tried to shoot 200 more today. Seems after 5-6 shots I'd get no bang & the round had a light primer strike. Then another 4 or so shots then the same light primer strike. I picked up the rounds up & tried to fire them again & some did work & some did not.

I have eliminated the ammo & mags as a cause. Ammo & mags function fine in another AR. Using lake city M855/pmags.
My BCM upper works fine on the same lower so I don't think it's the lower, buffer or spring. (Using a Spike T2)
While cleaning it today I did notice the gas tube seems to be a little loose. I can wiggle it inside the upper slightly
Could the firing pin be bad? The BCM BCG has at least 2K on it maybe a little more.

I did not try to swap BCG's from the BCM to the HBAR.

Also I found if I shot slow like 1 shot every few seconds, I'd have no malfunctions. But as I increased fire speed so did the light primer strikes & no bang.

If your rifle fires a single round, ejects and locks back on the empty magazine..........

Try a heavier buffer. If you are running a carbine buffer, step up to a H. Your rifle upper may be a bit over gassed and the bolt is bouncing. A heavier buffer will hold the bolt forward when chambering and prevent bolt bounce.

Already have the spike T2 which is heaver then an H.

The upper is cleaned & lubed properly, & I run it wet. As always.
I'll measure the FP.

The lower is fine.
What gets me is 1 day it's fine for 300 rounds & then it acts up the 2nd time out.
Doesn't make sense.

I did see the firing pin tip seems a bit more rounded then the others I have.   I did use this well used BCG in a new upper as well so maybe there's something there. Sending it back to BCM for replacement.

I may be confused ,but did the upper come with a BCG that was head spaced to it  or did you not have a BCG with the new barreled upper & use a BCG (used ) from another barreled upper ?
Swapping BCG to different uppers can have odd results .Once a bolt, that is head spaced to an barreled upper & its wore in ,it may not work correctly in another barreled upper . Now the swapping of just the BC is another story.

The new upper did not come with a BCG. I had the BCG in another upper I sold off & kept it for this upper. The BCG has at least 2K rounds on it from the other upper.

I really didn't think it mattered.

Yes, it can matter. Did you buy a replacement firing pin yet?

No, ordered a new BCG from BCM. I'll let them warranty the 1 I have if there's anything wrong with it. They said it will need to be looked at. Meanwhile I need the AR up.

SOB! I guess that was it. I installed a brand new BCG. 1 mag went fine, 2nd mag rapid fire a few rounds didn't go bang. Same problem. Changed the buffer from a spikes T2 to a H2 buffer.  Problem gone. Ran 6 more mags through rapid & slow fire.

That's nuts.