Interesting topic. The US Military has done extensive testing on the effect of crimping on accuracy. Lee advertises that their factory crimp die enhances accuracy. The military tests indicate otherwise. Therefore, crimping may not be a good idea to be universally done to every round of ammunition the handloader assembles.
When is a crimp needed? With Revolver ammunition to prevent bullet creep from recoil which could lock up the cylinder. Typically done with a roll crimp built into most seating dies.
With autoloader ammunition (pistol and rifle) to prevent bullet setback when it is being "slammed" into the chamber. Since pistols typically headspace on the case mouth, a taper crimp must be used.
One other situation requires crimping.....Firearms with tubular magazines such as the Winchester 94 where bullet setback could result from the compressed magazine spring putting pressure on all of the rounds in the tube.
Bolt action rifles. Crimp not needed and maybe detrimental to accuracy. Therefore, in most cases and IMHO, the Lee factory crimp die is a waste of money. I have one in my entire inventory. It is .30-30, which I actually only load for my nephew. While I own a Model 94 that was made in 1963 (the last of the pre-64s), it sits in the back of my gun safe and retains its virginity, having never been fired. A lot of factory ammunition is "factory crimped," but only because there are many autoloaders out there in the popular calibers, e.g., .223, .243, .308, .30-06, etc., and ammo manufacturers cannot assume it will be fired in bolt actions rather than autos.
