Hrmm...
I experienced a similar problem not long ago.
It ended up being my gas block. I had a local gun shop install my free float rail (should of just done it myself) and they didn't tighten my gas block down.
First outing it functioned fine.
The second outing it short stroked almost every round.
Third outing it was effectively a single shot rifle. My gas bock had worked itself forward so NO gas was getting back to receiver.
With that said... check your gas block.
Also... if possible have a friend stand next to you and watch the bolt as you fire. To rule out short stroking. If you're getting full motion from your BCG then it's probably not a gas problem.
didn't see this post below.
Quoted:
Thanks for the replies.
Forgot to mention that I tried the "one round in mag" to see if bolt locks back on empty mag, and it performed properly with all magazines (7 total), so that does not sound like a short stroke to me, which is what I was starting to think it was. The fact that it would cycle fine (albeit randomly) at times leads me to believe that it is not short stroking by definition.
I saved one round that got "passed over by the bolt" and you can see two small "dings" and a scratch the length of the casing whre the bolt hit it when passing over. The mags were seated properly in the magwell.
Also, gas key is tight.
Maybe it's just the magazines, which by the way are brand new. The two metal ones are NHMTG 30 rounders. I took them apart and they appear to be brand new. However, the rounds seem to fit really tight in the metal mags. It is very difficult to strip rounds by hand. Not sure if that is normal.
So, here's what I'm going to do (in order):
1. Soak the BCG in lubricant ( I thought I did this, but maybe not enough). - couldn't hurt
2. Check the gas block for misalignment and gas tube for crimps. (it's brand new, and as it did fire through two full 20 round mags, I doubt it's obstructed) - Also doubt it.
3. Try different ammo (although I don't see how this is the problem, as the rifle did have hiccups of succesful firings, see #2 above) - Ammo can play a surprisingly large role in the function of a rifle
4. Try some different mags and label current ones when testing (could be that the two that fired all 20 rounds properly are the only two good ones I have.) - Should always have marked mags imo
5. Change to a shorter buffer spring. - Have you tried moving your entire upper to a different, this would rule out the buffer spring, buffer & tube
6. Take the lemon back to the gun store and pitch a fit. (there are probably other steps to try before this, but a new rifle shouldn't cost me too much money to fix...the gun store's gonna have to come off some parts). - possibly, I learned about the AR platform from fixing my first AR that I bought.
Please comment on my comments, as I need all the help I can get here.