First of all, we're talking about a smoothbore shotgun, not a rifled barrel slug gun. The Mossberg Silver Reserve is an O/U shotgun. As such, he'll necessarily be firing either Foster or Brenneke style rifled slugs, not sabot slugs, so there is no issue of having the slug shift in the sabot. Thirdly, the forcing cone on shotguns is very long and the rifled slug is very soft and designed to deform easily to obturate to the bore and then through the constriction of the choke, making the transition into the bore completely safe, especially since the slug will have a good 1/2-3/4" running start at the forcing cone. As for accuracy, nobody's expecting anything great from a rifled slug anyway, so the difference between a 4" group and a 6" group at 100 yards will probably be lost to anyone who chooses to hunt deer with a shotgun traditionally used for upland game.

I know plenty of guys who put half a dozen plus BOXES of 2 3/4" rifled slugs (both Foster and Brenneke style) through 3 and 3.5" chambered smoothbore semi-autos every deer season, usually as fast as they can pull the trigger, and never have any ill effects. If it was dangerous, most of them would be missing a body part or two by now.

Mike