Specs:
Upper- DPMS MK12 5.56, 18", rifle length gas system, FA carrier, low profile gas block, M4 receiver/barrel
Lower- DPMS with A2 stock, rifle buffer and spring, RRA match trigger (2 stage)
"2nd lower" - DPMS, carbine buffer/spring, standard carbine buffer weight. Has a rounded hammer installed (for use with TacSol .22 upper)

After shooting the rifle 30-40rds, the rifle seems to short stroke and FTF. Once it fails, it happens every 2-3 shots, and the bolt does not stay open on an empty magazine. The bolt carrier is dragging on the magazine, and especially on the unfired ammo in the magazine. After firing a full 30rd (NHMTG) magazine on a clean rifle, the front lower edge of the bolt carrier, as well as the bottom of the carrier, have brass marks and even brass shavings from the unfired ammo in the magazine. Also, EVERY successfully ejected case hits the brass deflector causing the mouth of the case to look like a D, and all of the brass has scratches on it from the bolt carrier.

I have ran several different magazines through this rifle (PMAG, NHMTG, C Products, Troy), and several different types of .223 and 5.56 ammo (mostly Federal XM193); all of which function properly in my factory M&P15 - and the mk12 still won't function properly once its warm. I have also tried the mk12 upper with a "2nd lower" with a carbine buffer system, and it ran exactly the same.

On a side note, the mk12 upper fits VERY loosely on ANY lower I have paired it with. I've read that most AR15's are 'loose' in this area, but this seems excessive. If I hold the lower and twist the upper back and forth, it makes a very audible 'clunk clunk' noise. I haven't been around AR's very long, so maybe this is way off... But could the base of the upper receiver be machined too much, causing a lack of clearance between the bolt carrier and a loaded magazine?

Also, I clean the rifle after every trip to the range. I always use lots of Slip2000 oil on important areas, but this last time the entire bolt carrier was dripping wet. Seemed to work better than usual, but the problems are still there.

What are some items I should check? Am I overlooking something? Is there maybe a height specification for the upper receiver? Thanks for any help and information!

After a trying a few different things, the rifle performs as it should now. Thanks for all of the help!

I scrubbed and scrubbed and scrubbed any moving parts in the upper (and inside the upper), and used different lubricants and cleaners (Break Free CLP and Slip2000 seem to work well).

I purchased a 800 grit chamber hone from FlexHone, and LIGHTLY honed the chamber, and cleaned the chamber/bore thoroughly. Tested the chamber with go/no-go gauges before and after, and passed both times. I have read multiple problems with DPMS mk12 barrels having a rough chamber, I just wanted to be sure.

I found a few small gas leaks at the gas block and at the front of the gas tube. The amount of residue seemed to be the same as my fully functioning M&P15, however.. Maybe the rifle gas/buffer system is more sensitive to gas pressure than a carbine. Being that the (aluminum) gas block has been removed, machined to fit under a handrail, and again removed and installed multiple times to diagnose this issue, I replaced the gas block with a complete/assembled steel gas block and gas tube from Wilson Combat (adjustable, since I also plan on using a suppressor in the future). Since I had the adjustable gas block, I also opened the gas port one size just to be sure I wouldn't have to remove the gas block again. I also 'sealed' the block with some blue Loctite.

ALSO, I found 2 of my magazines hanging up on the bolt. For some reason, they only drag on THIS rifle.

After some tuning with the adjustable gas block, the rifle functions properly now with any ammunition I have fired through it, and holds the bolt open on an empty magazine. I have read that the mk12 system is picky with ammo, and is suggested to only fire 5.56, but most of my 5.56 (including heavy bullets) wouldn't cycle. I won't own a rifle that is picky on ammo, I want it to be able to shoot anything I may have to put in it. So, finally, this rifle is worthy of being in the safe. And it is AMAZINGLY accurate, with any weight bullet I've put through it.

Thanks again for the input, and all of the information on AR15.com! It is a great resource, and I wanted to post my results in case anyone has similar problems.