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The length of the shell refers to the length WHEN FIRED!
In layman' terms - The inside of the barrel, the chamber is long enough to accomodate a 3" fired shell lin your gun. Naturally this allows you fire the 2 3/4" length shells as well.
FWIW I finally have a 12 bore shotgun that has a 3" chamber. I have yet to fire one, for all my years I have felled any and everything with 2 3/4" shells.
[Exception is in .410]
Tidbit: To determine the dia of a pellet , subtract the shot size from 17.
For instance to find the pellet dia of #6 shot : 17-6= 11 therefore #6 shot is
.11 in dia.
T4