To avoid building an SBR it must have a bbl length of at least 16" and an overall length over 26" with the buttstock extended (if folding). Some states, notably Michigan, have their own regs regarding folding stocks and such.
Heavy enough bolt and recoil spring will make .45 work fine in blowback. All the subguns chambered in .45 have been blowback with the notable exception of the first Tommy guns. Even those were eventually converted to blowback operation with the M1 model.