Never fire a 3" shell in a gun chambered for 2 3/4"...
but I can't tell you how often ....I hear in gunshops ...where a guy is shopping for a used shotgun and someone takes a 3" shell unfired ....and says, sure, see it fits into the chamber - its ok ????
( its not ok )....like someone mentioned above, shotshells are measured after they are fired for length !