Thinking of building another AR in 300 blackout or 6.8spc. I am leaning towards the .300 since it uses the same bolt and mags. Any of you have either or these cartridges? Will probably end up deer hunting with the rifle.
300 Blackout=300 Whisper=300/221. There are very minute dimensional differences in the 300 Blackout compared to the 300 Whisper, enabling the 300 BO to be a factory round w/ SAAMI specifications for chamber pressure and chamber specs. I've got a 12" 300/221 Contender barrel I've used for deer hunting. It's a fine deer hunting round. In my Contender, I get 2250 fps from the 125 gr Nosler Ballistic Tip.
I'm not into the AR platform, but it I was, the 300 BO would seem to make more sense than the 6.8.
One other point re: the 300 BO. If you intend to shoot the 220 gr, subsonic bullets, be sure you get a barrel w/ the appropriate twist for this.