I have built 3 different ARs, all in 5.56/.223, a couple in carbine length and one in mid length, all 16" barrels. I still have a stripped lower in the cabinet and am thinking about doing something different. Maybe 5.56/.223 in a 20" barrel or even a 300 Blackout. Not sure. What all calibers can be built on a standard lower?
The other guns I build were all fairly inexpensive with parts from PSA and shoot great but thinking a long distance gun could be cool, too. I've seen some videos of someone shooting coyotes with an FLIR scope and may do that if I can scratch up the dough.