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Revolver Cartridge Crimp

How-To
Crimping of Revolver Cartridges



 

The crimp begins with the belling ( flaring) of the case mouth to allow the bullet to be seated easily without shaving metal from the sides of the bullet.

When using the case expander die, the the second die in a three die set for almost all hand gun cartridges. ( Lyman sells single purpose expander dies they call an M die. )

 

 


Remember that the expanding operation is cold working the brass. The brass is being expanded, stretched. This will work harden the brass and make it brittle.

Eventually the case will split at the mouth.  Almost all revolver cartridges fail in this way. Most splits are longer than the image.

These splits are not dangerous and you will not know it happened until you eject and inspect the cases.

split case moutn

Click on the image to expand it.

When adjusting the expander die, set it to expand the mouth of the case as little as possible to allow the bullet base ( base only ) to enter the mouth of the case.

 

expanded case moutn

Click on the image to expand it.

Note that only about 1/16 inch of the base of the bullet will enter the mouth of the case. It is possible to expand the case mouth deeper, but that will also expand it wider and seriously shorten the life of the case.

You should be able to easily insert the bullet base into the mouth opening but not to push it down into the case.

Expanded Case Mouth

Click on the image to expand it.

The case on the left has been resized. Note the case walls form a perfect cylinder.

The case on the right is the expanded case. Note that the expanded part is visible but barely noticeable unless you are looking for it. The expanded part is sometimes referred to as the "bell" or the flared mouth.

Case mouths

Click on the image to expand it.

Another way to determine if your expanded case mouth is expanded properly.  The resized case mouth will just barely slip into the flared opening of the expanded case.

That is JUST BARELY. It will not enter as far as the base of a sized bullet.

Now that you have seen all the images and read all the instructions on how to do it correctly and accurately. This is how I really do it in real life.

Back the expander die stem out of the die until you can raise the ram to the top of the stroke without touching the expander plug.

With the ram at the top of its stroke, screw the expander stem down into the die until it enters the mouth of the case and continue screwing it down until you feel the stem try to expand the case mouth.  Lower the ram and feel the mouth of the case. There should be no belling. Screw the expander stem into the die about a fourth turn and run the case into the die again. Pull the case out and feel for the expansion, repeat this until you can just feel the tiniest flare of the mouth of the case. Now it is time to try a sized bullet for fit. You may need another quarter turn, maybe two.

This takes about half as  long to do as it does to describe.

Expanded Case Mouth

Click on the image to expand it.

The resized bullet will enter the mouth of the case easily for just a few thousandths. In the photo, the driving band protruding from the case mouth is the driving band. The driving band is also called the base band. It is approximately the same width as the band visible just above it.

Expand the image and you will see that the flare at the mouth leaves a narrow space between the brass of the case opening and the lead of the bullet band.

IMPORTANT NOTE:
This bullet is sized .357. An un-sized, "as cast" bullet would measure .360 to .361 and would not enter the mouth of this case.

I do not recommend loading un-sized bullets.

Expanded Case Mouth

Click on the image to expand it.

Seat the bullet deep enough into the case that the mouth of the case is as close as possible exactly even with the top of the crimping groove.  Please expand the photo to get a better view of what this looks like up close.

Also not on the expanded image that the mouth of the case has not been crimped. At this time, we are adjusting the seating depth, not loading finished cartridges.

You will also note the Lee Liquid Alox lubricant.

Correct seating depth

Click on the image to expand it.

The first stage in adjusting the crimp die is to back the seating stem completely out of the die. You must be absolutely sure it does not contact the nose of the bullet and seat it deeper into the case.

Next raise the ram to the top of the stroke with the bullet as far as it will go into the die. Screw the seating/crimping die into the press until you can feel it make contact with the case mouth. When this happens you will also usually see the cartridge case move slightly in the shell holder.

Remove the cartridge and look closely at the bell. Use a glass if you have one. If there is any sign of flair remaining, replace the cartridge in the shell holder and screw the die into the press no more than 1/8 of a turn. Continue this until the case mouth looks perfectly cylindrical, with no visible flaring or belling. ( Expand the image now to see what this looks like. )

FYI:
One full turn of the die will advance it 0.071 inches.
One fourth turn of the die will advance it 0.018 inches.
One eight turn of the die will advance it 0.009 inches.

Adjusting the crimp die.

Click on the image to expand it.


When Zero crimp has been achieved, turn the crimping die into the press one fourth turn. 

I use RCBS dies. This will likely differ slightly with different dies.

The one quarter turn crimp is what I call a "Light
Roll Crimp".

This is my standard crimp for all revolver cartridges up to and including the .357 Magnum.

Adjusting the crimp die.

Click on the image to expand it.

I recommend you stop here.

If you turn the die one forth turn further, you will get a heaver crimp. Stronger or deeper seems to convey the meaning better but "heaver" is the accepted term. 

I would not argue with anyone who found this crimp to be necessary but in my experience all it will accomplish is shorter case life.

Adjusting the crimp die.

Click on the image to expand it.

One more quarter turn gets you to this level of crimp.

I have seen people crimping this heavily but I have never done it. I expect it would begin to degrade accuracy because it is not compressing the second driving band. 

Adjusting the crimp die.

Click on the image to expand it.


This brings us to four fourths, one full turn of the die past zero crimp. This means the mouth of the cartridge case has been forced into the crimp ring approximately 0.070" 

At this level we are doing serious damage to the bullet.

Adjusting the crimp die.

Click on the image to expand it.


Here is the bullet pulled from the above cartridge. You don't need to expand the image to see the damage. 

We have created a lead Coke bottle.

The diameters of the driving bands from front to back measures:

Band 1  .357
Band 2  .341
Band 3  .357
Band 4  .357


I do not know how, or if,  this will affect accuracy. It is easy to say it has just created a new lube ring out of a driving band. 

I do know what it will do for case life and it is not good.


Adjusting the crimp die.

Click on the image to expand it.

 

 

 

   
   
   
   
   
   
   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


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