The nice thing about the AR platform is the possibilities of caliber should be controlled by only four things, to keep it as basic as possible (I know, more than this, but being basic please):
1) The diameter of the bolt face
2) the length of the bolt
3) the strength of the action
4) the weight of the buffer and the strength of the recoil spring together
You could "custom one-off" build a direct impingement AR in any possible caliber... I think some like the .338 and .50 would need muzzle devices though.