Better question, is: why discourage it at all in the first place?
Cutting the barrel down means the mag tube has to be shorter as well.
My HD shotgun already only has a 5-shot tube. And I can't extend the tube due to the design (Stevens Ithaca clone), and even if the design allowed it, I could only get 1 more shell in there and still remain at 18" tube length.
Cut shotguns mean fewer rounds and less accuracy. No choke to keep the buck spread tight. No front sight to use when shooting slugs. I'd sure hope for that guy to be at the mall shooting I was a victim of, rather than the guy with an 18-20" security shotgun with a 9 shot tube that is even with the tip of the barrel.
Unlike box magazine fed firearms, tube fed reloads are slow. Yes, there are magic fancipants guys that can reload shotguns crazy-fast. But they don't do mass shootings and more than Rob Leatham or Jerry Miculek do.
Let's say he cut it to 12". He should only be able to fit about 3 shells in there.
Yeah, it's a devastating weapon close in from sudden concealment against a surprised security guard. I guess it's a good way to trade up to a high standard capacity pistol or rifle. But for a shooting spree, it's quite limited.
