Re: Sellier & Bellot Buckshot . . . Thoughts?
The shell can be labeled as a 2.75 because when fired it still fits in a 2.75 chamber. The shell is roll crimped, because of this when it is fired in a 2.75 chamber the shell material that unfolds stays in the chamber and doesn't flow into the forcing cone/throat area. If you were to fire a 3" cartridge in a 2.75 chamber you would have pressure issues as the folded material moves into the forcing cone/ throat and constricting it as the wad and shot enter it. This causes a constriction while the shell is still near peak pressure. This is bad. So for all intents and purposes, it is a 2.75 shell because the measurement is actually done post firing. If you notice in the post above the shells measured less than 2.75 when in the unfired state.
It's all fun and games, til someone shoots your bobber!