If you are lucky enough to own a Remington 7615, do not destroy it's increasing value by making it into a wildcat platform.
What is wrong with the .223 as a 'hog' gun?
If you think it is too light, get a 7600 in .308 or .30-06, the carbine version.
I cannot understand the infatuation with the .300 blackout. I routinely download .308 loads to subsonic, but you can't 'upload' a blackout to 60,000 PSI loads going 3,000 pfs (you can with a 30-06)
Unless you are wedded to the AR platform, I see the 300 blackout as a step backwards.