peters, it doesn't matter whether the chamber is cut a wee bit undersize or was originally intended to be a 2 1/2" chamber. The point is that it is well proven that lower presure loads can be safely shot in short or tight chambered guns. The issue of short chambers is pressure increase due to the hull mouth opening into the forcing cone. It has been shown that low pressure loads have a trivial increase, plus the low pressure to start with gives you room for the increase.
I agree with Brown and 2-p. Load 7/8 oz (1 oz loads if you insist) at 1100 - 1150 fps and about 7000 psi. If the gun is is in shape to shoot, these will easy on the metal due to the low pressure and easy on the wood and on you due to the low recoil. You can shoot them by the bushel basket, too.
To answer you question directly, you need to take a series of exacting chamber diameters at exacting depths and draw out the profile. The "2-p method" of sidelighting the inside of the barrel and measuring from the chamber face to the shadow of the forcing cone with a steel rule will settle the issue of chamber length. Note that shotgun chambers are far less critical of dimensions than are high pressure rifle chambers. Shotgun pressures can be effectively sealed with materials (paper or plastic) far more compliant than the brass cases need for high pressure sealing.
