Back when I was a kid hunting with my dad, he had an old Eastern Arms (Stevens) .410 single. Chamber had been lengthened from 2 1/2 to 3". I saw Dad roll some bunnies way out there with a 3" load of 6's. Looking at his gun today, forend taped to the barrel, paper thin at the muzzle, I'd be hesitant to shoot 2 1/2" shells in it.