Today, one can walk into a gun shop and purchase, for instance, a .22, .38 or .44-caliber handgun. Most firearms are built to accommodate one size round only. Here’s what would happen if the manufacture of today’s standard-size rounds were outlawed, and .21, .37, or .43-caliber rounds took their place: Eventually, gun owners would run out of the old ammo, and their weapons would become paperweights.