Is this gun a 12 ga or 20 ga. Going by the 2 1/2" chambers, I'm guessing 20 ga. Anyway, if you like to reload, and are not in a hurry to do it, cut your own shells to size and roll crimp them. Whatever load fits into a 2 3/4" 6 or 8 fold crimp will fit into a 2 1/2" roll crimp.
I am doing some right now in 16 ga. that has 2 9/16" chambers, using Remington black hulls, Unique powder, 7/8 oz shot, Rem. SP16 wad, one .030 overshot card, roll crimp using old fashioned hand roll crimper. Problem with this load is that in order to get a nice roll crimp, I had to cut hull to 2 3/16". Without cutting to this size, you would have to had more overshot cards to fill space.
