To my knowledge, there were no 2.5" 1897s produced; it should be chambered for 2.75... sort of.
It will take, and fire, modern 2.75 light loads without problem. I know mine does. The issue is that the chamber is too short, and the plastic hulls open into the forcing cone, causing increased chamber pressures. You can tell by trying to rack a fired shell through it. The action will not close on the fired shell. I'm guessing this is why the gunsmith said it was a 2.5" chamber. Long story short, it has to do with the crimp in paper hulls taking less material to form, which in turn correlates to a shorter overall length when fired.
The best solution isn't to go buy a new shotgun, but rather to get your chamber reamed out. Should cost less than a hundred bucks, and will allow your gun to swallow new, plastic-hulled 2.75" shells without an issue. Don't be afraid to use it to shoot as you would any other gun; the things are built like tanks out of higher quality steel than you're liable to find in most new guns. Mine has lasted god knows how many years of military duty and decades more of sporting use and it still busts clays with a regularity that depresses my Remi 870-wielding friends.
