Case Gage? Crimp Gage!

By Duane Thomas

If there’s one area in which many handloaders’ final product, re: autopistol rounds, could be improved, it’s in crimping the case mouth. The case mouth must be expanded to accept a bullet, then after the bullet is seated, the flared case mouth must be smoothed back down, otherwise the oversized case mouth will prevent the round from fully chambering. The most common error among people reloading autopistol cartridges is overcrimping, i.e., the case mouth curves in at top, cutting into the bullet. In this article I will share with you an easy way to set your crimp die to avoid overcrimping.

But first, let’s discuss why we want to avoid overcrimping. There are two common sorts of crimp:

1. Roll Crimping: the case mouth rolls in at top. This is what we generally see on revolver cartridges, especially heavily recoiling revolver cartridges, as it’s necessary to prevent bullets jumping forward in the cases under recoil, possibly protruding from the front of the cylinder and tying up the gun. Many revolver bullets have crimp grooves formed into the bullet body, partway along its length, for the roll crimped case mouth to fit into without deforming the bullet.

2. Taper crimping: case mouth flare is straightened out to the point the round will chamber and that’s about it. This is what we typically see for autopistol cartridges.

NOTE: There are two types of people in the world. (1) Those who do not like numbering things. (2) Those who like numbering things. Obviously, I’m the second type of person: I like numbering things.

There is a myth going around – and it is a myth, I assure you – that applying a heavy crimp to an autopistol cartridge prevents bullet setback during feeding. This is untrue. What prevents bullet setback during feeding is friction between the sides of the bullet and the interior of the case, also the fact that, when a bullet is seated, its diameter is greater than the interior diameter of the resized case. This means the bullet slightly expands the case down as far as it’s seated, then underneath that the case angles back in, giving the bullet a full circumference little “shelf” on which to sit.

Problems with overcrimping are threefold: (1) It cuts into the bullet, deforming it and degrading accuracy; (2) on plated bullets it can cut through the plating, leading to pieces of plating sheering off and clogging the chamber; (3) pressing the case mouth in at top can actually cause the “shelf” lower down to straighten out, therefore it stops supporting the bullet, and, far from preventing setback during feeding, makes it far more likely to occur.

So, we know we must taper crimp our autopistol rounds so they’ll feed. We know we don’t want to cut into the bullet while doing so, or compromise the “shelf,” all we want is to straighten out the case mouth flare. How to accomplish that? The common “wisdom” goes something like this: “Measure the width of the bullet you’re loading. Measure the thickness of the case at the mouth and multiply by two. Add the first number (bullet width) to the second number (mouth thickness on both sides of the loaded round), and the resulting number is what the overall width of your loaded rounds should be at the case mouth.”

This approach comes with a few problems: (1) Different brands of cases have different dimensions in various areas, including case mouth thickness. A number calculated using one brand of case might be too high or too low for another brand. If you reload mixed brass – I certainly do – this is a concern. (2) If you can hold a round steady in one hand, and dial calipers in the other, while measuring across the round’s diameter riggggght at the very tippy top of the case mouth, you obviously have much steadier hands than I do.

The solution, I find, is to use a case gage as a crimp gage. A case gage is a cylindrical piece of metal that’s had drilled into it, longitudinally, a hole equivalent to a tight match chamber. After handloading a cartridge, insert it into the case gage to check its overall dimensions. If it’ll fit into the gage, it’ll fit into a chamber. One way you can tell a serious handloader, they have case gages for any cartridge they load.

Using a case gage to check crimp: (1) Start with the crimp die on your reloading machine backed out, to the point it will not completely get rid of case mouth flare. (2) Pump the handle, run a loaded round up into the crimp die. (3) Remove that cartridge from the machine and insert it into the case gage. Does it drop easily into place? Does it fall free smoothly when you turn the case gage over? No? (4) Tighten down the crimp die slightly, like a quarter turn. (5) Repeat steps 1-4 until the round drops easily into the case gage and drops free easily when the gage is inverted.

Congratulations, you have just set your crimp die to taper crimp your autopistol rounds the absolute minimum amount necessary to get rid of case mouth flare and allow smooth feeding, without cutting into/deforming the bullet, or compromising the “shelf.” And because you are basing the amount of crimp on the overall dimensions of the loaded round, you will still be turning out rounds to that spec no matter what the case mouth thickness. Such a deal.