Another option is 7.62x39. Ammo generally available.
Mine is 10.5" from PSA.
Montana Marine, couple years back I was given A Colt 7.62x39 A2 upper 16" with 3 20rd?mags . I just pulled it out, I’m going to slap it on to one of my lowers I see how she shoots. If I like it maybe I’ll pick up a pistol upper,I think that would be a nice truck gun.