The difference between a roll crimp and a taper crimp is just that. A roll crimp rolls the end of the case inward into a cannelure or groove in the bullet. It is formed by forcing the case with bullet seated, up into a die which has an inward curving radius that literally rolls the brass, leaving an outside radiused curve on the case mouth. Look at a factory revolver round under a ten power glass.
A taper crimp is formed by forcing the case with seated bullet into a gently inward tapering die, which progressivley straightens out the mouth belling necessary for seating bullets, and then, as the crimp increases (and mouth diameter decreases) the case mouth is forced laterally into the side of the bullet. Too aggressive a crimp will in fact, weaken or sever the bullet jacket.
What to use? For revolvers, a roll crimp provides consistent bullet pull, and in moderate to heavy recoil, the roll crimp prevents the bullets of rounds in the cylinder awaiting their turn from being pulled through inertia. For most semi's - 9mm and .40, and .45ACP among others - a taper crimp allows the case mouth to come to rest at the abrupt end of the chamber, thereby preventing the cartridge from entering the leade, and causing excess pressure. Insufficient taper crimp impairs cycling, too much allows the case to slip past the chamber's end, neither are good. Most manuals have a diameter showing the ideal outside dimension of the loaded case, at the mouth. It is a good place to start.
