When you roll crimp a standard length new 2 3/4" hull, you only use 1/16" of hull length for the crimp, approx. Therefore, when doing this, you obtain a 3" hull volume. This only works when using the rto crimp, NOT folded crimp which takes over 1/4" length of the hull. You can put a rto crimped 2 3/4" hull next to a factory 3" folded crimp shell and they are the same height or length. If you like, I have pics that I can email you of this comparison which might help you understand better. My email is email@example.com
Also when I load the very same load in both the 3" rto crimp shell and the 3" folded crimp shell, I load them the same except I load the folded crimp shell with one less grain of powder. By doing this, it gives them both the same pressure. Thus, the rto crimp drops pressure and velocity some, unless of course you account for it by adjusting the powder charge.
If you desire to load rto crimped 2 3/4" shells, using 2 3/4" new hulls, you will have to trim them shorter than standard over all length.