I've never been really impressed with 300blk. I know it's more my fault than the round but I've had a few pigs run off with 300blk subs. Almost all the pigs I've shot with 308 fall over dead.
This statement confuses me. Are you comparing apples to apples here? Are you talking about 300blk subs not working, but 308 subs do work? or are you comparing 300blk subs to 308 supersonic?
Also, if you build a 308 SBR, are you going to suppress it? I would think that 308 supersonic loads would be pretty harsh on a can, although I could be mistaken.