Well. . .it might help if you give us a list of calibers you'd like instead of the .223.
In your subject line you reference .300 Blackout. That's one of the easier ones to convert. The easiest way is to just buy a .300 Blackout upper. It would pop right on and off you can go!
If you want to go the difficult way, but probably the cheapest: buy a .300 BLK barrel and gas system and swap it out. Everything including the upper receiver, bolt, bolt carrier group, lower receiver and all the internals will work.
If you're going to silence it, you may need to play around with your buffer, a bit, but that's no biggie either.
As for other calibers, we'd need a bit more info as to what you'd like to do with it.
The Professor
