CRIMPING BASICS

TYPES OF CRIMPS

ROLL CRIMP: Bends the case mouth inward creating a curved shoulder.

• Commonly used on revolver rounds or other calibers that headspace on the case rim.
• Crimp should align with crimp grove on the bullet being crimped into place.
• Most standard die sets (both rifle and handgun) are supplied with the bullet seat die that applies only a roll crimp. Many reloading for rifle ammunition will not use this feature (or apply a very light crimp) which is controlled by the depth the die is installed in the press.

TAPER CRIMP: Tapers the case into the bullet.  The case is tapered to a smaller diameter at the mouth and angles the brass mouth into the bullet creating an edge on the cases. Generally this task is accomplished by using a Taper Crimp Die, usually this is sold separately.

• Commonly used on cartridges that headspace on the case shoulder.  Bottle-neck cartridge design is the most common case.  Semi-auto calibers also can be tapered crimp.
• Crimp does not need a crimp grove.

ADJUSTING A SEAT DIE TO CRIMP

The basic CRIMPING process with the Seating die is accomplished by screwing the die into the press until contact is made from the case mouth, turn in half turn more for a light crimp up to one full turn for a heavy crimp. Never use a heavy crimp on ammunition that headspaces on the case mouth, this is typical of straight-walled pistol rounds (examples: 9mm Luger, 45 ACP, 450 Bushmaster)

• If the cartridge case are not trimmed to a uniform length a consistent crimp is impossible to achieve with regular dies. Using a CRIMP DIE can solve this problem, still consistency in case length should be a goal to achieve.

WHAT SHOULD BE CRIMPED?

• Any rifle round intended to be used in lever actions and pumps.  Generally, if it has a tubular magazine the bullet must be crimped to prevent movement.  Firing of the rifle tends to dive the bullet into the case of rounds stored in the magazine.
• Revolver ammunition, here excessive recoil begins to pull the bullets from the case.
• Ammunition loaded for hunting should be crimped, especially in rifles that generate heavy recoil.
• Keep in mind any crimped round will rank between difficult to impossible to pull the bullet from the case. Smaller the bullet, the more difficult the task!

Richard Lee explains the reasons to use Crimp Die, the full story can be found on pages 38-41 in MODERN RELOADING-SECOND EDITION which will be centered around the LEE Factory Crimp Die.

• Improved accuracy, can be applied to bullets seated at any depth, your reloads are less sensitive to variables related to powder lot and primer make.
• Richard Lee advocates that a crimped bullet is more efficient since the bullet will not travel until pressures inside the case can overcome the crimp, roughly 250 PSI more is needed to move a crimped bullet when compared to an un-crimped bullet. This higher start pressure insures a more uniform pressure curve and less velocity variation.
• Factory-loaded ammunition relies upon a crimp to provide a uniform start pressure, a process that can be duplicated when using a crimp die as your final stage in the reloading process.
• There is a reverse theory on crimping and that pressures can be controlled by precise bullet seating depth, which is correct.  Keep in mind that this process is mainly reserved to a particular load fired in only one rifle, it is not interchangeable with other rifles sharing the same chamber size.  This is why factory ammunition is not produced in such a manner, only the individual Reloader can create such “custom ammunition”. Seating depth theory is controversial topic with many correct answers, nearly every reloading manual and shooting forums discuss this, usually in conjunction of increasing accuracy.

Every die manufacturer produces crimp dies, at duckcreeksportinggoods.com we keep on hand mostly crimp dies from Lee Precision. The reasons are based on quality, selection, and pricing. 

There are three basic styles of crimping dies on the market:

1-TAPER CRIMP, Every tool and die manufacture makes them, found inside the shopping pages here at Duck Creek are the Lee in mainly handgun calibers . Click here for example. Redding brand Taper Crimp dies can be special ordered, generally cost around $35.00. The Redding’s are divided into bottleneck cartridges (204 Ruger to 350 Remington Magnum) and straight wall calibers (30 M1 to 454 Casull).

2-CARBIDE SIZE & CRIMP combination die, again can be of Taper or roll design.  This tool will POST-SIZE a loaded round to guarantee everyone will positively chamber with dependability by removing any bulge that may have happened during the reloading process.  At the same time, a crimp is applied (usually a roll crimp). Click here for a example.

3-COLLET CRIMP relies on a collet ring inside the die to create a uniform crimp the case mouth, strong enough to squeeze the case into a bullet forming a crimp groove if none are present on the bullet. Click here for a example. A original to only Lee is the FACTORY CRIMP DIE (FC), which applies the crimp by the use of a collet. This type of die applies a segmented crimp, similar to what is found on a hydraulic hose by applying pressure on 4 points. Click here for a example. The FC die is standard in Lee 3-Die PACESETTER rifle sets.

Applying to great of a crimp can damage ammunition, it will bulge the brass beyond repair. It is important to follow the directions that are supplied with the tool or use the guidelines mentioned earlier in this post if no instructions are available. Using a caliber specific “AMMO CHECKER” is a good way to verify what you created will properly chamber, I find the Lyman Handgun Gauge be be helpful, click here to view.