So I tried out the new 300blk build today.  It worked well for the first 20 rounds of gorilla supersonic ammo and then decided to stop ejecting.  The bolt would not move until I pulled the charging handle back.  I tried about ten more rounds with no better luck and even had 2 rounds that the extractor did not grab.  I decided to try out some remington supersonic ammo but had exactly the same issues.  I then swapped out the 300 blk upper for a 5.56 upper with a PSA bcg and it ran flawlesly so I decided to put the 300 blk upper on the lower but swap out the midwest industries bcg in the 300 blk upper with the PSA bcg from the 5.56 upper and continued to have the same problems.  The weird portion comes in that I started feeling what I could have sworn was gas on my wrist which was near where the barrel and reciever met, the amount of gas I felt seemed to get worse as the round count got higher.  The upper reciever is aero precision and the barrel is whoever brownells use as their brand.  This is the third upper I have built but have never had this happen before.  Does anyone have any suggestions?

300 blk upper:
Aero precision upper reciever
Midwest Industries BCG
Brownells 300 blk 10.5" barrel with pistol gas length
Spikes tactical gas tube
Midwest industries gas block
Midwest industries handguard
Surefire flash suppressor

Is the gas block a set screw or clamp on type?  It sounds like it came loose and shifted forward on the barrel. This can expose the gas port on the barrel, hence the gas on you forearm.  And of course if that gas isn't going down the has tube, the gun won't cycle.

The gas block is a set screw and this is the first barrel that I used that did not have the dimples for the screws, so I will take a look at it today and see if thats the problem.

AFAIK the most common problem for your description of the symptoms would be a broken gas screw.
There have been a number of shifted gas blocks reported too.

Sounds like the gas block is loose.

I finally found some free time to check out the rifle and the gas block did come loose while I was shooting.