We’re going to use a weird analogy to illustrate what crimping does and does not do. It’s kind of like inserting a wooden dowel into a hole drilled in a block of wood.
This Hornady taper crimp die removes the flare from the case mouth. Image courtesy of Hornady.
SERIES:
- Part 1: Reloading: Want To Reload Your Own Ammo? Basic Questions to Consider
- Part 2: Reloading: The Basic Process
- Part 3: Reloading: The Gear You’ll Need and What It’ll Cost You
- Part 4: Reloading: Brass Cleaning and Preparation to Load
- Part 5: Reloading: Brass Resizing
- Part 6: Reloading: Trimming Cartridge Cases
- Part 7: Reloading: All About Primers
- Part 8: Reloading: Priming for Success
- Part 9: Reloading: Powder, Propellants, and Pressure
- Part 10: Projectiles: Materials, Weights, and Styles
- Part 11: Seating and Crimping Bullets
- Part 12: To Crimp or not to Crimp
- Part 13: Final Inspection and Packaging Tips
Crimping gets a bad rap.
Just as we sometimes tend to fix construction mistakes with a hammer rather than taking the time to properly fit parts, we tend to view crimping as a way to “fix” a less than perfect bullet seating. The problem with crimping is that it sounds so logical. The word “crimping” kind of implies the process of locking a bullet in place.
We know that a bullet has to be firmly seated in a cartridge. If a bullet is too loose, it can move during recoil or in extreme cases, fall out of the cartridge altogether. If a bullet in a magazine or cylinder moves partially out of the cartridge case during recoil, it can jam up the works. In a revolver, that may prevent the cylinder from turning. In a semi-automatic, that cartridge might cause a feeding jam. On the other hand, if a bullet gets pushed farther into the case, it can increase pressures to catastrophic levels. As the bullet gets pushed in, there is less internal case volume, and when you touch off that round, less volume means higher pressure. How much higher depends on the specific load and amount of bullet setback.
As a result, it’s important to make sure that projectiles are seated firmly and properly in the cartridge case. The gotcha is that crimping is not the way to do this, with a few partial exceptions that we’ll discuss in a bit. The short explanation is that crimping is not what keeps a projectile firmly seated, case neck tension is, and those two things are not the same. Likewise, crimping doesn’t create or even restore case neck tension. Let’s explore the whole case neck tension thing a bit more.
Case Neck Tension Explained
A good way to understand the concept of case neck tension is to drill some holes in a 2×4 board.
Imagine we have a standard piece of 2×4 lumber. We’re going to drill some holes and stick wooden dowels in them. In this analogy, the board is our cartridge case, and the wooden dowels are the projectiles. Our goal is to make sure that the dowels are stuck in those holes in the 2×4 really tightly so they can’t be pulled out too easily. For this example, assume that our dowels are exactly 1/4” in diameter. We can drill the holes exactly that size, a little smaller, or even a little bigger if we like. Likewise, when we flare case mouths (analogous to drilling the hole in our 2×4) in cartridge cases, we can do it a little or a lot.
For starters, let’s assume we drill a 3/16” hole. That’s a hair smaller than the dowel diameter, so we’ll have to force the dowel into the hole. As we do this, the dowel will compress a bit, and the size of the hole will expand just a hair. The result is a very tight fit. It’ll take a lot of work to get the dowel into position, but once it’s in place, it’s going to take a lot of force to remove it. You can think of this like case neck tension. Since the dowel and hole are tightly mated, there’s a lot of friction between the two surfaces – the exterior of the dowel and the interior walls of the hole. If we seat a bullet the same way, but with much closer dimensional variances, there will be a similar result. The bullet surface and inside of the cartridge case will be tightly joined, and you’ll have to overcome a lot of friction to pull that bullet out. Make sense so far?
If we want to “seat” this wooden dowel so that it doesn’t move in or out, drilling a slightly smaller hole will do the trick.
Now let’s imagine the opposite scenario. If we want to take the easy way to “seat” the dowel in the hole, we can drill a larger hole, say 5/16” and the dowel will just drop into place. You’ll be able to remove and reinsert the dowel with ease since the sides of the dowel and interior of the hole basically don’t contact each other. If you want the dowel to be tightly “seated” you can just “crimp” the dowel into place, right? If you stick the 2×4 into a giant vise and compress it from the sides, it should press the sides of the hole against the dowel, thereby making a really unusual wood crimp. I think we can all agree that’s not going to work out too well. No matter how much you squash the sides of the 2×4, even to the point of mashing the sides of that 5/16” hole firmly against the dowel, it’s not going to hold. Compressing the sides of the hole against the dowel simply won’t create the same degree of friction as our first example.
Can we agree that “crimping” this dowel into place with a vise isn’t going to be effective no matter how much we squeeze the vise?
With most materials, it’s impossible to recreate the tension created by stuffing a larger object into a smaller hole by pressing on the sides of the hole. When you seat a bullet into a cartridge case that has too large an interior diameter, the same thing happens. You can squeeze that case all you want, and the bullet will never be tightly seated. In fact, you can squeeze so much that you actually reduce the tension, making the bullet fit even looser than before you started crimping. Or, you can deform the bullet, leading to all sorts of wonky shooting results.
There is one “partial” exception – roll crimping – and we’ll talk about that in a minute. However, even in that example, it’s important to get proper case neck tension even before you think about applying that roll crimp.
Before we discuss the two most common types of crimps, we need to understand the concept of headspace. It’s a big topic all on its own, so for now, just think of it this way. A cartridge needs to sit in a chamber or cylinder in just the right position away from the firing pin. It needs to stay put so that when the firing pin strikes it, the impact doesn’t simply push the cartridge forward. Different calibers use different means of controlling headspace, and that impacts that type of crimp that can be used.
Straight wall pistol cartridges like 9mm, .40 S&W, and .45 ACP use the edge of the cartridge case mouth to control headspace. If you look in the chamber of a pistol barrel, you’ll see a little ledge. The case mouth jams up against this ledge, preventing the cartridge from moving forward. If the case is the right length, headspace is controlled.
Straight wall pistol cartridges like these use the case rims for headspace, so only a taper crimp can be used.
Revolver cartridges like .38 Special, .357 Magnum, and .44 Magnum headspace using the rim on the base of the cartridge. That rim is what stops the case from moving farther into the cylinder.
Most revolver cartridges use roll crimps like these. If you look closely, you can see where the case mouth is slightly turned into the bullet cannelures.
Many rifle cartridges headspace using the shoulder of the cartridge case. That part where the case narrows to projectile diameter jams up against its mate section inside the chamber to position the cartridge correctly.
Roll Crimping
Roll crimping refers to the process of pushing (rolling) the cartridge case mouth edge into a groove called a cannelure that’s cut into the bullet. The edge of the cartridge case “digs” into a groove in the bullet, helping to prevent forward or backward movement. You can use roll crimping on cartridges that headspace on the rim or shoulder of a bottleneck cartridge case because the cartridge case mouth edge is not relevant to proper headspacing. Simply put, a “ledge” on the cartridge case mouth is not needed. You’ll also notice that many military cartridges use some variation of a roll crimp as well. Since those headspace on the shoulder, a roll crimp can be used to help make sure the bullet doesn’t move under recoil. Cartridges destined for use in machine guns often use this type of crimp.
Bottleneck cartridges that use the shoulder for headspace can use taper crimps, roll crimps, or mashed-up roll crimps like the one in the center.
Taper Crimping
Taper crimping really isn’t crimping at all, at least in the commonly understood sense of the word. Think of taper crimping as the process of gently squeezing the cartridge case mouth back into its normal position once a bullet is seated. As we discussed earlier in the series, before seating bullets into straight wall cases like 9mm, you need to ever so slightly flare open the cartridge case mouth to make room to start a bullet seating. After the bullet is seated, the taper crimp gently pushes that flare back to original dimension. Taper crimping really does not do much to increase the tension between the case and the bullet. Consider the wooden dowel example we discussed earlier. In fact, too much taper crimp pressure can squash the projectile, loosen brass case neck tension, and make that bullet less secure than before you started.
So, back to the basic question of “to crimp or not to crimp,” the answer is yes – with qualifications. The type of crimp will be determined by the cartridge you’re reloading, so that’s an easy decision. The important thing to remember is that crimping is not a corrective technique for sloppy seating. Finding the right degree of case mouth flaring will help you achieve proper case neck tension so the crimp can do what it’s supposed to do and nothing more.