If you have a heavy recoiling setup like a lightweight .357,a good roll crimp helps but you also need a proper case grip on the bullet,thats where the lyman dies shine in that area. The case has to sized small enough to grip the bullet but not so much to deform a cast bullet.The stepped mandrel in the bell die provides the proper tension,if the bullets try to pull out the mandrel may be chucked in a drill and reduced with emery cloth a 0.001 or so to increase tension. Lee and RCBS do not do this unless they have changed their dies recently.