I feel like it would just make more sense to regulate shit based on actual math related to the stopping power of a gun and not the dimensions.
Like, if you've got a 45, a mag/barrel can have no more than this much ammo and it can't fire faster than x times per second and can have only so much accuracy out to y range or something like that.
People get too fixated on the fucking shape of the gun and not the bullets that are the ones killing things.