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Old September 25, 2012, 01:04 AM   #11
Lost Sheep
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Join Date: January 24, 2009
Location: Anchorage Alaska
Posts: 3,341
Quote:
Originally Posted by wogpotter

What crimp? (.357 S@W magnum.)
Fairly new to reloading pistols so I've been using the crimp seater die that came with the set. I now realize I want to get a sepereate crimp die, but which one?

Roll crimp?
Taper crimp?
Factory crimp?

I intend to use it with both Hornady XTP JHP's & plated bullets so I'd like something that is suitable for both as I don't want to buy multiple dies.
Taper crimp.

Taper crimps are used on cases that headspace on the case mouth (semi-autos, mostly). A roll crimp does not provide good contact of the case mouth with the headspacing shoulder in the chamber.

You COULD taper crimp for a revolver. In fact, it would be a good idea with plated bullets because they generally don't have a crimping groove.

You NEVER want to roll crimp into a plated bullet because if you cut the plating, bad things can happen.

If you adjust carefully, a die intended for a roll crimp can do an acceptable emulation of a taper crimp. Just don't roll into the sides of the bullet.


Roll crimp

Roll crimps are only used for revolvers or cartridges that headspace on something OTHER than the case mouth (almost always the rim).

There are two things that keep the bullet stationary in the case (until firing). The crimp is one. The other is friction with the case walls. The tighter the case walls grip the bullet, the more certainly the bullet is held, and this friction provides greater force than crimp (I am told).

This bullet tension is important not only to keep the bullet in place during handling and recoil of other cartridge's going off, but to provide that the pressure buildup upon ignition follows a good, consistent pressure-time relationship.

So, here's the deal.

When you bell the case mouth, you allow for insertion of the bullet, but no more, lest you have a loose fit. When you seat the bullet, you are pressing the bullet into the neck of the cartridge against the friction of the case mouth and the sides of the bullet. The more the better, generally. When the powder ignites and pressure builds up, the case mouth is expanded by the pressure and lets the bullet go, but only after a proper amount of pressure is present for good combustion. Too little friction and the bullet moves too soon and poor combustion results.

The roll crimp is pretty much just insurance and a little extra retention force if you have really heavy loads. "Gilding the lily" so to speak.


Factory Crimp

Factory Crimp? What's a Factory Crimp? Lee calls one of their dies a Factory Crimp Die (FCD), but their naming conventions leave a lot to be desired. But you should be aware that Lee's FCD performs two functions. It crimps (roll crimp for revolver cartridges and taper crimp for semi-auto cartridges unless you specifically ask Lee Precision for something different). The other function is to size the finished cartridge to ensure it is within SAAMI specifications. This sometimes causes problems because of squishing the lead bullet down in size, loosening the friction grip the case neck has on the bullet.

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