The Lee Factory Crimp Die crimps the bullet in place more firmly than aby other tool. It is impossible to buckle the case as with regular roll crimp dies. Trim lenght is not critical. Tests demostrate that even bullets which have no cannelure will shoot more accurately if crimped in place with the Lee Factory Crimp Die. Factory ammunition is often more accurate and better able to withstand rough handling because the bullet is firmly crimped in place. A firm crimp improves accuracy because pressure must build to a higher level before the bullet begins to move. This higher start pressure insures a more uniform pressure curve and less velocity variation. Even powder selection is less critical. Until now, handloaders seated the bullet too touch the rifling to achieve similar results. this is not always possible not desirable. The Lee factory Crimp Die is included at no extra charge with Lee Pacesetters Dies. It's just another added value that only Lee Dies offer!