Consider a .45 acp. Ballistically the same as .300 shooting subs, ammo is cheaper, and lots of defensive ammo available. Several different options for guns in .45acp.
I've got ar pistols in both 300 and 45. I have found that while 45acp can be had a little cheaper the defense ammo is almost a wash. It is much cheaper to build a 300blk, good 45 uppers are in th 750+ range, and magazines are astronomical. the 45 is more fun, but the 300 is more economical. It would take a lot of ammo to make up the difference in initial cost and accessory cost. There are some 45 pcc's out there not of the ar variety, but lots of them are finicky and most are relatively low capacity.
If I were running the 300 for home defense, even with a can, I would be running supers. The can will get rid of the majority of the damaging sound. Having both gives options, but I'll take 30 rounds in somewhat compact magazine over 21 in a footlong stick hanging out the bottom. The extra umph of a supersonic 300blk is nice too, I've laid a buck down at 50ish yards with one which builds confidence in the ability of the round for home defense.