Historically, taper crimp dies were designed for cartridges that headspace on the case mouth i.e. autoloaders 9mm Luger, 45 ACP, etc.. They squeeze the sides of the case against the bullet, but do not disturb the case mouth.
Roll crimp dies were intended for rifles, or revolvers, that headspace on the rim. They "roll" the mouth of the case into the crimping groove. Some of the slow burning powders require a "heavy" roll crimp. Whatever that is. The problem came about when some reloaders thought roll crimping was like whiskey. If a little was good, a whole lot was better.
Wrong on both counts. Extremely heavy roll crimps in standard roll crimping dies sometimes caused the case to bulge out, which led to chambering problems. This is because the bullet seating and crimping operation are performed in the same die at the same time.
The Redding die provides a roll crimp, but also employs a very close fitting die body (profile) which prevents the case bulge, and also gives sort of a taper crimp instead of a roll crimp alone. You must crimp and seat in two separate operations.
I think these are a great innovation, and use them in my .357, .44 Mag. .454 Casull and 45/70. I hope this answers your question.
