Remember, shotgun chambers are measure with the crimp OPEN. A 2 3/4" cartridge is closer to 2 1/2" loaded and extends the extra 1/4" upon firing. Even though a standard length round will fit, the question is will it fit when fired?
Try dropping a fired hull with the crimp extended as much as possible. If that will drop in smoothly you "may" have a standard length chamber. If the barrel is not marked with a chamber length, you really should have it checked by a competent gunsmith. He/she can also give you a good idea of the overall safety level of the gun with modern ammo.
In life, strive to take the high road....It offers a better field of fire.
"Robo is right" Fuzzball