Can you use 2and three quarter inch shell in 3 inch chamber?

Answer 1

The answer is ABSOLUTELY NOT ADVISED. The shotgun "chamber" is the length of a breech section of larger diameter that accomodates the wall thickness of the shell (to provide a smooth transition from the interior of the expanded shell to the barrel of the gun). Although the unfired longer shell can be loaded in the shorter chamber, the crimp cannot fully open when fired... it will be pinched in the narrower barrel diameter in front of the chamber. That causes increased gas pressures during firing and could possibly damage the gun. (Newer guns should handle this slight overpressure safely but you may feel a slight increase in recoil.)

You can, however, go the other way around and safely use shells shorter than the guns chamber (eg a 12 gauge can safely use 2 3/4 inch shells in 3 inch chambers). There is some debate about the effect of this mismatch on shot pattern. Some credible long standing studies actually indicating a more uniform shot pattern from using shorter shells in longer chambers.