If you want a larger bore try 50 Beowolf.
You can still buy a 50BMG upper Both single shot and magazine models. Or roll your own.
6mmAR 40 dgree turbo and the 22AR woukld top my list of wildcats. they use Grendal brass. the 6mm aproaches 243 Win MV.
the 30 RAR necked down to 6.5mm or 6mm would be another.
I started on a 308x1.5 barnes AK a but droped it when I relized that I could just light load any 308 Win based round and beat any thing made to fit in a AR length mag Unless I went to 284 win brass and shortned it and necked it down.
I have a 17-223 and its a lot of fun would make a good AR or AK round.