Just had a look in Lyman's 5th Edition of Shotshell Loading book and the heaviest 'slug' listed for 2 3/4" hulls is 525gr., their .69 calibre slug that looks like a Diabolo Pellet. This one fits into plastic wads, usually the Red Winchester WAA12R wad, but others are listed as well - for smokeless loads.
Other loads, including fiber wads is their 475gr. Foster slug.