Ok I have built one 300 blackout - functions perfect.  A buddy wanted a 300 and provided his new in the box DPMS Oracle ar as the base firearm.  I removed the barrel, gasblock, etc.   I installed a new MAS 300 1/8 16" barrel, new low profile gas block, carbine length gas tube, and a  freefloat handrail.  All went together nicely.  Bought so of the 185 gr subs and some 125 gr supers.  Loaded 1 in mag then chambered round and fired.  The spent casing was ejected but the bolt did not cycle back enough to be held open.  Same with the supers.  I made sure firearm was unloaded and empty and I made sure the gas block was properly positioned over the port in the barrel.  Tied again same results.  I then loqded mag with 6 rounds each and it ejected and picked up the next round about 60% of time.  I seperated upper and lower and the bolt moves as it should and seems to lock up fine, etc.  When back together it does seem hard to  rack the bolt and charging handle.  Has anyone had an issue  similar?  Should I send the lower back to DPMS to check?  I checked my other rifles and they sure do not seem as tough to rack back.

Any tips?

Sounds like it needs a little breaking in.  Since it seems to be getting better as you shoot it, make sure it is lubed properly and hand cycle with the charge handle about 200 times while you watch TV then try it again.  Also make sure you have a standard carbine spring and buffer.

Thanks.

For what it's worth, my builds seem to always like being run really wet for the first hundred rounds or so. Two of my 300blk builds had similar problems to yours before they broke in.

What buffer weight?

Here is slow motion of mine cycling. The subsonci is weak and barely cycles...