[edited] I need to say that I'm not fond of the AR platform at all, for any caliber, not a purist, just old school. 30-06,308 heritage. I really like Springfield Amory's M1A Scout Squad but price is a deal breaker.
Part of the info I found that convince me the 300BLK was worth a look is here 300BLK
and the drawings are here SAAMI
The 7.62 can load up to 200 gr and I would experiment in HARD cast lead bullets along with currently available jacketed offerings..
AFAIK the twist rate would have to be 1 in 10" - 1 in 8" with 1 in 8" being better for heavy 308s or 310s
all 300BLK loads work with the standard mini14 223 mags
For Scotty, if ARS is Accuracy Rifle Systems in Odessa TX, I sent them an email on 300 bbls about a week ago asking, because they don't have the caliber listed, and have not gotten a response yet.
And thanks, I've looked over Accuracy Systems and like their stuff so far.
I don't know about 7.62X40 Wilson but have looked at 300 Whisper and 300-221, 300 Fireball but are all 'wildcats' and I wanted to keep some costs in check so wasn't liking wildcats so much.