What are Taper versus Roll crimp differences?

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roll crimpers are most often used with cast lead bullets that have a groove for the case mouth to 'roll' into...45colt in particular.
In the mag tubes of leverguns it keeps the bullets from becoming deep-seated under recoil as one round bumps against the other.In handguns it keeps the bulles from creeping either forward or backward in the case.

taper crimp 'squeezes' the case sides against the bullet,but does not curl the mouth of the case in,it just lays flat against the bullet.Nearly always used on jacketed ammo.

a lot of folks don't taper crimp if they have not flared the case mouth AND the neck tension is firm enuff. I like to apply a light taper crimp alway , because neck tension from case to case is never perfect..far from it.
 
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Another difference is in the design of the cartridge. When a cartridge is rimmed, and designed to be headspaced from the rim, often a roll crimp is specified. On auto-loaders, or where the cartridge is headspaced by the case mouth, a taper crimp is utilized merely to squeeze the case mouth back against the bullet and allow the round to chamber without catching on any "lip".

If the round is headspaced by the shoulder, as in a rifle round, any crimp can be used according to one's choice but to use a roll crimp on a "case mouth headspaced" round can cause length variations that may or may not lead to problems.

Lee usually states that "the die supplied for a particular cartridge has the correct taper method".

As for the LEE Factory Crimp Die, it is possible to put enough "crimp" in a case to actually create a groove much like a cannelure. Use this feature wisely.
 

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