Barrel length is a factor that is considered for whether a firearm is prohibited or not, in conjunction with other characteristics. In the case of a shotgun like a Benelli M1 or M3, if the barrel gets too short, the firearm could wind up being restricted, and if it gets shorter still, it could become prohibited. It depends on exactly how the barrel becomes shorter and exactly what the length is. It varies depending upon the exact circumstances, but essentially, the shotgun barrel length is considered in conjunction with other factors to arrive at a determination of classification.
On December 6th, 2022. See this statement in context.