Well, I wandered over to my LGS (local gun store, for the more refined amongst us) and came home with a Hollenbeck--an original Three-barrel Gun Company drilling. It's 12ga/12ga/32-40, and I can't find any indication the 12's are 2-1/2 inch--but since it has Damascus barrels, I'm loading my own BP shells, starting fairly light at 68gr of FFg under 7/8 oz. of shot. I had some steel shot 3" 12ga wads lying about, trimmed them to come fairly even with the shot load, and crammed a 14ga overshot card down the throat of the hull to cap them--can't get to the range to try them out today, but by tomorrow I'll have some 32-40's loaded up (light) and we'll see how she shoots. One caution I noted early on, with primed (only) hulls: I don't have a sensitive enough trigger finger to shoot the rifle barrel without also touching off the right 12ga barrel. Could be my own deficiency--but then, the gun is over 100 years old, and shows it.
She's not much of a looker; the buttstock has been replaced, the forearm fiddled with, and the current front sight came off something else, but I'm obliged to care for her now, and feed and exercise her commensurate with her age. I'll try to measure up to the task. More as it develops! windy

Welcome to the party. Way to start out.