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In a shotgun, the front of the shell "unfolds" into the chamber behind the forcing cone. If the shell is too long (2 3/4" in a 2 1/2 chamber for example), the front of the shell will open into the forcing cone after the shell is fired. That will raise pressures a bit, but since shotgun pressures are fairly low, and steel barrels, even those made in the 1920's, are plenty strong, the "problem" is almost never noticed. Now whether the longer shell will seat depends on the design of the chamber and the forcing cone. Usually, a shell 1/4' or less too long will seat, though sometimes with difficulty. A more common problem in an autoloader or a pump with a shell that is too long will arise with the carrier, which might or might not handle shells of varying length.
As to ejection ports, I don't have any measurements at hand but usually they are made longer than absolutely necessary to handle any variation in the ejection cycle. So it would not surprise me if a 2 1/2 or 2 9/16 gun would eject 2 3/4 shells.
Jim
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