In a way this sort of applies...
Recently some customers have been getting me to build smokeless muzzleloaders based on modern bolt action rifles. Not my thing, but it pays the bills. They want to shoot thin jacketed bullets at bore diameter which evidently are soft enough to bump up if loaded heavily enough. And of course they want a muzzle brake, since these rifles have tremendous recoil. So I have learned that a brake with a fairly large diameter bore can accommodate a turned funnel which not only allows them to get the powder charge in, but also functions as a false muzzle and allows the bullet to be started straight with no effort using a fitted short starter. Of course you wouldn't want a brake on your slug gun, but the concept is somewhat similar to what you're doing. Just make enough engagement area to keep everything straight and you should be just fine.