Oh, BS! Millions of 2 3/4" slugs have been fired through both 3" and 3.5" chambers without any negative effects. The taper between the chamber and bore on a shotgun is very slight and won't cause any problems at all. Not to mention that the 835 is overbored to 10 gauge (.775") bore, so that there is no constriction whatsoever between the chamber and bore on that model shotgun.

Basically, shoot whatever length slug you want. Better yet, try a few different slugs to see which on shoots the best in your shotgun. My concern with the overbored barrel would be that if the slug doesn't obturate enough to seal the very large bore, you'll get gas leakage around the slug causing poor accuracy and worse than normal leading, but only your gun will tell you if that's a problem.

Mike