AR15 platform; in the AR10 I would just do the 358 Win.
I considered the 556 case blown out to 35 cal, but that's a straight wall case, basically a rimless 357 Max, and would have to headspace on the case mouth.
rost, what bullets are you getting to expand subsonic from the 300? I've done a lot of experimenting with 30 cal subsonic bullets, and the only bullets that will expand significantly use a large hollow point. Even round nose cast bullets with pure lead noses don't expand, unless they hit something hard. Maybe on bone, but not in meat. I do cast my own hollow point bullets for that, but the 35 cal just works better and easier.
