I have several calibers 300wsm, 270wsm on the LR308 lowers and 22lr , 9mm , 6.8spc on the AR15 lowers.
Project pistol caliber guns being built are .357sig , 40s&w , 10mm auto all waiting for BCG's.
The 6.8spc is my favorite and most accurate caliber but ammo is scarce and expensive. I will end up selling one gun or an upper , dont shoot it much anymore can't afford to feed 4 hungry mouths.
The 9mm is the fun plinking gun but the range is 50 yards. Past 75 yards and its a 6 to 12 inch pattern.
I will also be building a 300aac blk, got the barrel but that is it.
The WSM calibers are a novality , rounds are $2+ each and I had to taylor the handloads to 95% power or they wont cycle. But the range is 1000 to 1200 yards and velocity is from 2900 to 3900fps. Also made heavies at 200gr and 220gr in the 30 cal.
I always tell people to do a caliber thats different but feasible. Try something like a 38 super in an AR pistol now that would be a fun toy and cheap to feed!