The American shooter is so ingrained with the idea that a shotgun can be purchased off the shelf and used, much to the chagrin of our English brothers, who tried, unsuccessfully to teach us, that you can't do it that way!!!!
It will stun the average shotgunner how well they can hit and hit succcessfully when that firearm they're carrying around actually fits.
I learned through a fitted (bespoke) Spanish sidelock double while I was in the Navy over in Spain.