old gun, new shells

[Clark McCombe]

Are Manufactures supposed to release the pressure for their various loads?
I've been using Fiocchi 12 ga 2 3/4 " 1oz 1250 fps 7.5 shot shells in an $800 Trojan which I found out yesterday, when I got my little feeler gauge, only has 2 1/2" chambers.
I immediately switched to using RST 2 1/2" shells.
The Fiocchi cost $120 a flat
the RST $230 plus shipping.
I can:
Practice with an old Remington 1100 I have, which I would rather not do.
Have the chambers lengthened on the Trojan.
Continue to use the Fiocchi in the 2 1/2" chambers.
Or be distracted by the thought that every time I pull one of the triggers it will cost me a dollar if I switch to the RST, and accept the fact that using the old gun I like will cost me.
Your thoughts please.....