I have both 300AAC and 7.62x39 as a bigger bore addition. The 7.62x39 came first as an optional use for the ammo stock for the SKS(s). By handloading, I've been able to move that upper into suitable deer taker class.
The 300AAC was just a wild hair. I had an upper and the barrel was the only one Brownells had on hand the day I ordered. It seems to have some potential as a deer gun for Grandsons and has shown much better accuracy than the best I've been able to achieve with the x39 besides it doesn't require additional mags.
Brass can be reformed from .223 if you don't mind the extra effort.
While it's likely a good idea for familiarity training, I refuse to pay twice as much for a 22lr upper vs a simple effective 10/22.