Originally Posted by
Pizza Bob
LTC is exactly right, but allow me to clarify/differentiate. When the term "crimp" is used in reloading, it can refer to two different types of crimping. As pointed out, a taper crimp is used on cartridges that are supposed to headspace on the mouth of the cartridge (where the bullet enters the case). As LTC said, a taper crimp removes the "belling" and actually swages the case against the bullet, while still allowing a clearly defined case mouth upon which to headspace.
The crimp that your manual was warning against would be a roll-crimp. A roll-crimp is used on cartridges that are rimmed (headspace on the rim rather than the mouth), so a sharply defined case mouth isn't critical. You generally roll-crimp revolver cartridges. The case mouth is actually rolled into the bullet cannelure or crimp groove. This prevents bullet set-back during recoil and allows for better powder combustion.
You want to taper-crimp your 9mm cartridges. You also want to insure that the actual case length does not exceed the max allowed. In point of fact, most rimless cartridges that theoretically headspace on the case mouth, actually headspace on the extractor, so case length isn't all that critical as long as the max isn't exceeded.
HTH. Good luck.
Adios,
Pizza Bob