To Crimp or Not to Crimp — That Is the Question


Recently, a student (who is new to reloading) asked me about crimping handgun cartridges. To be honest, I have been reloading for almost longer than I can remember and all of my procedures and dies are set. That made answering his questions somewhat difficult. Difficult because his questions jumped around, and I could not put my answers in a logical teachable order.

I quickly realized that answering random questions would not work and decided to start at the beginning to make everything — as to the how and why — logical and understandable. Admittedly, one area that most shooters could use a better understanding of is the subject of crimps. So, let me try to add my two cents to the subject and get the hate mail started.

A slight roll crimp on a light-recoiling .38 Special practice round.
A slight roll crimp on a light-recoiling .38 Special practice round.

To begin with, I believe we all understand that when the cartridge is fired it expands, releasing its hold on the bullet and allowing it to begin its journey to ‘who knows’ where. When the reloading process starts, the first procedure is to return the case to its original size by resizing the case. The next step is to expand (bell or flare) the mouth of the case, so it will allow you to seat a new bullet.

Once the new bullet is seated, the case must be squeezed to its original dimension. That way, it holds the new bullet in place and can be once again chambered properly in the firearm. The process of returning the flared case mouth to the size that will hold the bullet in place is referred to as ‘crimping.’

Questions that invariably arise concern which type of crimp to use and how much crimp is correct. It is of course possible to ‘over crimp’ when your crimp die is set incorrectly. That is something you most definitely want to avoid.

Roll Crimp

There are basically two common handgun crimps that we will discuss. The first is the roll crimp. With the roll crimp, the case mouth rolls in at the top to securely grip the bullet. The roll crimp is what we see most often on revolver cartridges, especially those revolver cartridges with excessively heavy recoil.

The roll crimp is necessary to prevent the bullets from moving forward in the case under recoil. If that happens, they could protrude from the front of the cylinder and jam the ability of the gun to function. Most bullets — designed to be fired in revolvers — will have a crimp groove formed into the bullet body. You’ll see this at some point along the bullet’s length for the roll-crimped case mouth to curl into, without damaging the bullet.

Brass cartridge cases collected from a shooting range
All cartridge casings are not created equal! Just because the caliber matches, does not mean the casings have the same case mouth thickness.

Almost all manufacturers offer seating dies with a built-in roll crimper — especially for revolver cartridges that utilize a roll crimp. To adjust the die to properly roll crimp, raise the seating die body so the case is not crimped when the press is at the top of the stroke. With several prepared dummy cartridges (no primer or powder), adjust the seating stem so it seats the bullet at the proper OAL, and seat a bullet in each of the 6 cartridges without any crimp.

Remove the seating stem. With the ram at the top of its stroke and a dummy cartridge in the shell holder, lower the die down until you feel contact with the case mouth. Next, lower the ram, and adjust the die body down approximately 1/4 of a turn.

Run the cartridge back through the die and examine the crimp. If more crimp is called for, adjust, and repeat until the crimp is correct. Then, tighten the lock ring and have at it.

Headspace infographic

You will then be good until you switch bullets. A different bullet will probably have a different seating depth, and you will have to adjust the seater itself until you get the cannelure centered correctly on the case mouth.

Tapered Crimp

The second type of crimp found on handgun cartridges, specifically those intended for pistols, is the tapered crimp. With the tapered crimp, the flare of the case mouth is straightened, so the round will chamber. This allows friction alone to hold the bullet in place.

When discussing pistol-cartridge crimping, we must remember headspace. Headspace, on a straight-walled cartridge, is defined as the distance between the face of the bolt and a point in the chamber that prevents further forward movement of a cartridge. All straight-walled, rimless pistol cartridges, headspace off the case mouth. That makes the tapered crimp essential. Remember, pistol cartridges have a case diameter that is slightly larger than the bullet, and the chamber has a shoulder for that case mouth to rest against as in the illustration of the .45 ACP.

Infographic of an exaggerated Illustration of the “shelf” that is formed to prevent set back.
An exaggerated Illustration of the “shelf” that is formed to prevent setback.

When crimping for auto pistols, one common mistake is over crimping — especially on the larger calibers. Most believe applying a heavy crimp to the cartridge prevents bullet setback during feeding. That is not correct.

Bullet setback — during feeding — is prevented by friction, between the sides of the bullet and the interior of the case. When the bullet is seated, its diameter is greater than the interior diameter of the resized case. This difference causes the bullet to slightly expand the case (as far down as that bullet is seated).

The case then angles in — underneath the bottom of the bullet — giving the bullet a little “shelf” around its circumference to sit on, preventing setback. The main problems encountered with over crimping:

  • Over crimping can cause the rim to cut into the bullet, deforming it and degrading accuracy.
  • With plated bullets, it can press through the plating. This will cause pieces of the plating to fall away and foul the chamber.
  • Pressing the case mouth too much — at the top — will cause the lower shelf (mentioned earlier) to disappear. Therefore, the lower shelf will stop supporting the bullet, making setback during feeding more likely to occur.

So, how do we determine the correct amount, of tapered crimp to apply, so the cartridge will feed and not cut into the bullet or compromise the shelf?

Dillon Precision 9mm headspace gauge
Just drop this 9mm Luger cartridge in the case gauge to determine whether it is correctly sized and crimped.

We need to remember some of the realities of reloading. Anything mass-produced will have variances. Proceed with the understanding that different brands of cases have different dimensions — including case mouth thickness. That means that any mathematical formula calculated using one brand of case might not apply to another brand. Likewise, if you reload mixed brass, the problem is compounded.

The Case Gauge

The simplest, or most elegant solution, is to use a case gauge. A case gauge is a cylindrical piece of metal that is machined with a tight, match chamber that is equivalent to the caliber it’s made for. After reloading a cartridge, insert it into the case gauge to check its overall dimensions. If it fits into the gauge, it will fit into a chamber.

By the way, all experienced reloaders have case gauges for all the cartridges they reload. The correct way to use a case gauge — to check crimp — is to first install the crimp die on your reloading machine, with it backed out to the point it will not completely eliminate the case-mouth flare. Then, run a round into the crimp die. Remove that cartridge from the machine and insert it into the case gauge.

Brass 9mm case fully seated in a case gauge
If it fully seats, it is good to go!

Does it drop easily into place? Does it fall free, smoothly, when you turn the case gauge over? If not, tighten the crimp down a quarter-turn. Repeat those steps until the round drops easily into the case gauge and drops free — easily — when the gauge is inverted.

Conclusion

That easy procedure will allow you to taper crimp your auto pistol rounds with the least amount of work. It will also allow smooth feeding, without cutting into/deforming the bullet, or compromising the “shelf” — regardless of the thickness of the case mouth or variance between shells.

I hope this eliminates some of the confusion and helps your understanding of the fine art of crimping. Stay safe, train often, and practice, practice, practice!

Do you have any reloading tips? How do you prefer crimping? Share your thoughts in the Comment section.


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