Well generally and I say generally because some companies are just off to do their own damn thing all the time.
But, when something says 2 3/4" or 3" that is measured after the fact. Meaning. It hits 2 3/4 or 3" after the crimp is unfolded. So if you measure it fired and its anywhere near that 3" mark. That's why, because even though the outside of the port is just larger than 3" the inside is what you have to worry about. The shell isn't clearing the chamber because the bolt can't go any further back.
Man, I have no idea how I know this or where I'm going with it, but in my head I'm making sense.