I'm wondering about the caliber more than the barrel length. I know the OK Law says .45 caliber and isn't 7.62x39 a much bigger bullet than .45 caliber? I'm not sure of the actual diameter of either. Far as that goes, a .223 bullet might be considered "larger" if the whole thing were taken to court. Guess that one would depend on the judge.
Caliber refers to the diameter of the projectile only. .30 caliber is less than .45 caliber even when comparing a .300blk to .45-120.