[WeeWilly]

There is no difference in how the actual crimp is placed on the case with the Lee Carbide Factory Crimp Die and the Dillon Die.

The difference in those two examples is the Lee has a floating collar, with a knob that allows you to adjust the crimp without loosening the lock nut that holds the die in the press.

In general, it is best to just return the case mouth to straight in a taper crimp style round (one that headspaces on its case mouth). When loading mixed headstamps with varying brass thickness, case lengths, brass hardness (annealed properties), bullet surface hardness, how much "line" that is left on the bullet will vary. It is always going to be a big of a circular dance, one where you are shooting for "average", the average being straight case mouth.

On the accuracy results of Lee versus Dillon (or any other crimp die), what you are shooting for in crimp is no deleterious effect. The problem with the Lee die (the one with the knob on top, not the collet style version) is the carbide sizing ring placed at its die mouth can impact the case mouth as it goes by (worst in straight walled calibers when loading larger than nominal sized bullets like say, lead). This sizing effect can be deleterious to accuracy.

 

[WeeWilly]

Not sure I understand all of the nuance of the current debate ahead of me, but a taper crimp does absolutely nothing for bullet hold. The most unfortunate thing in reloading was the taper crimp, being named a "crimp" because it is nothing of the sort when properly done. It is to de-bell the case mouth and nothing more.

Nonstandard effects can be created by "over-crimping" with a taper crimp die, for instance, when loading LSWC bullet profiles in a 45 Auto. Taper crimping past just straight can aid in feed reliability in some guns. Also, many gamers crimp past straight with their taper crimp die to force the case mouth inward slightly as a prophylactic measure against snags on feed in, again solely for feed reliability.

The above measures are trade offs, reliability for accuracy (and potentially leading in the case of lead), but have absolutely nothing to do with bullet hold increase, accuracy or any other thing usually attributed to these practices.

Brass has more spring than lead/copper under it. When you force the case mouth into the bullet with a taper crimp die, when it is removed, the case mouth springs back more than the bullet does, hence, taper crimp cannot add to neck tension, it only removes it.