In my opinion just buy the right size shells, a few companies make them now. As for the length of the roll crimp shells being shorter than the crimped shells, not true. If you stand both side by side they are the same length. The crimped shell has to use more of that length to form a six or eight crimp fold and it takes up length. I reload 20 ga 2 1/2" shells and I cut down Remington Premiers, STS, and Game Loads to 2 1/2" and load low pressure (under 7500 psi)low velocity, 1050 fps. I would never think of lengthening chambers and opening up forcing cone. That's my opinion and there are others out there that have done so.