RIDE, that's not the reason.
US laws mandate shotgun barrels to be a minimum of 18" long. This is typically measured by inserting a wooden dowel down the barrel until it hits the breechface and taking this measurement.
To legally possess a shotgun barrel under this length, it must be duly registered with the BATF and a $200 tax stamp purchased.
Not all states allow possession even then.
Without valid tax stamp, is considered possession of short-barreled shotgun (ie. sawn-off shotgun) = jailtime.