Some help with crimping

Most reloaders do not crimp rifle ammo unless it will be used in a tubular magazine or perhaps in a semiauto rifle. The crimp keeps the bullet from being shoved deeper into the case due to spring pressure in the tube magazine or the violent shove it gets into the feed ramp on an auto.

Crimping is generally necessary for revolver rounds for two reasons, first, to keep the bullets from coming out of the case during recoil of another round being fired and possibly jamming the cylinder, and second, for more consistent ignition of the powder charge and better accuracy. A proper crimp on ammo for a semiauto pistol keeps the bullet from being driven into the case during feeding and also makes feeding smoother in general.

A medium crimp is appropriate for most loads. A firmer crimp is needed for heavier loads. Most revolver ammo is loaded using a roll crimp, and most ammo for semiautos is done using a taper crimp. The roll crimp folds the end of the case into the crimp groove on the bullet, and the taper crimp squeezes the case walls down so that they grip the bullet more tioghtly, without folding the end of the case. You can use a taper crimp on bullets that don't have a crimp groove.

You can do an good job with the crimp feature of your seating die, but many of us nut jobs who are really serious about this stuff prefer to get a separate crimp die, such as the Lee Factory Crimp Die. It works well and allows you the satisfaction of having more stuff on your loading bench.

It's quite different. Look at any round of factory ammo and you'll see what look like short segments of the neck pushed straight in from the side, not rolled in around the whole circumference. The factory crimp looks like a series of dashes.

Roll and to some extent taper crimps work by pushing the loaded round into a crimping shoulder or angle. Because it is pushed in, an excessive crimp can crumple the case walls. A factory crimp (like Lee uses) exerts all its force perpendicular to the case mouth, so it not only can extert more force, but can NOT crumple the case walls. Jaws squeeze the case, and can even crimp to a bullet with no relief cannelure. Without damaging accuracy, I'll add.

To answer your general questions, higher bullet pull is good, whether created by neck tension or crimp. A "locked" bullet can not be pushed deeper (by a semi-auto or lever) or pulled out (by a revolver). It also significantly improves powder ignition and consistent pressure rise. Rifles and revolvers generally use a roll crimp, whereas semi-autos - which headspace on the case mouth - use a taper crimp. Cast bullets have a deeper relief cannelure/groove and can be crimped harder than jacketed bullets.

In summary, always crimp revolver and lever-action rounds. Usually crimp semi-auto rounds. With bolt and single-shot rifles, crimp is not mandatory, but it seldom hurts anything.