A lot of spin on this and other sites about if the short buffer for the AR platform works or not. I have researched this subject at length and helped develop one that works 100% of the time providing you do the following:
You must realize that you are working with a totally new system, it is not as forgiving as far as shooting any type of ammo but you can shoot several sizes if you pay attention to the following points ; Always break-in a new gun- ( 300-400 shots) if it is a 7.5 inch 5.56 1/7 barrel it is recommended that you shoot only 55 gr and 62 gr 5.56 ammo-no 223 ammo it is different. Learn how to properly lubricate your weapon. keep it clean especially the extractor and ejector do not lubricate the face of your bolt. Polish your loading ramp and locking lugs if you have trouble closing all the way in-to battery. This is not a short barrel gun it is a pistol so shoot it like a pistol with a finger grip over the magazine well to hold on with one hand and do a good push and pull just like a pistol. I might add that the M-16 bolt carrier works best with any AR pistol and is highly recommended for the 7.5 or 8 inch barrels.
Same for the 300 Blackout it is a little more forgiving than the 5.56, I can shoot 110,125,150,220 gr through the blackout 7.5 inch barrel 1/7 or 8 twist.
see www.syzygia-specwarser.com