Just bought my first AR. I got a 300 BO upper for it also.
The 5.56/223 functions flawlessly. The BO doesn't cycle enough to pick up a cartridge or lock back. I've tried about 4 different brands. Remington(gasp) cycles about 1/2 of the time, the others.......nada.
When I bought it(the BO), at a gunshow, I had them change the gas block to one with a rail so I could mount a front sight.
I'm thinking the gas block isn't lined up right. It looks square to the front rail, but maybe not located on the barrel properly. The gun shop that sold it to me is saying I need to change the buffer spring because BO doesn't produce the thrust/pressure needed. I don't know, but that sounds strange. I haven't read anything about that on the web.
Anybody got something to add to this or suggestions?