I have never had any experience with 2.5" 12 ga shells. But you spiked my interest so I did some quick research. You may or may not find it helpful. This was a post from another site, so take it with a grain of salt:
"Just be careful if you ever reload them. Many guns chambered for 2-1/2" or 2-5/8" were low pressure steel or damascus twist guns suited for blackpowder.
Originally 2-1/2"~2-5/8" guns used brass shells that didn't have crimped ends that needed room to open. Those brass shell loads were blackpowder. Today's 2-3/4" shells will fit into the shorter chambers because the shell length is measured prior to crimping. After top is crimped it is about 2-1/2". If you use one of those shells the hull wont have room to unfold and you could blow apart the barrel(s) right in front of the chamber."
Good luck and be sure to let us know how it turns out!