The guys nailed it, Chang. Use the short bbl with whatever shell lengths are listed on it, and use the 3.5" chambered bbl for the howitzer rounds.
Trust me, the shotload from a 2 3/4", 1200 FPS shell doesn't exactly bounce off perps and fall uselessly to the floor.
I've survived one war, three prison riots and a coupla Cub Scout overnight camping trips, but I dunno if I'm ready to deal with the blast and flash of a 3.5" shell from a riot bbl. A standard 00 duty round can be quite impressive when uncorked in low light.
And, if I had a dollar for every goose or duck I've taken with dinky li'l 2 3/4" rounds, I might be shooting Kolars instead of old 870s.
HTH....