If this has been beaten to death before, I apologize in advance. I don't spend much time here anymore. If 16-inch barrels are legal on rifles, as are 14.5-inch barrels with permanently attached flash suppressors, why don't we see 16-inch barrels on tactical shotguns? I've only seen 14.5 and 18-inch barrels. Is there such a thing? Is it prohibited? Or is it an overall length issue? Thanks!