Listening to reloading talk at the gun club, you'd think that crimp is part voodoo, part rocket science. How much, and how much for which caliber and application, seems as much random guesswork as it is recordable, testable mechanics.
Crimp, in this context, is the pressing of the case mouth back into place after seating the bullet — the final step in reloading your own ammo (well, the last step before inspecting said ammo). And to compound it, we have two crimps: roll and taper. Generally speaking, roll crimp goes on revolver rounds, and taper goes on pistol, but taper is so versatile it can work on both.
Taper crimp is much more forgiving than roll crimp, but it is also more subtle, so this article will delve into the aspects of taper crimp.
Your taper crimp die has a short section of cone-shaped taper at the top. You adjust how much crimp you are applying by screwing your die deeper into the press.
When you press the loaded cartridge in — you really should seat the bullet and taper-crimp as separate operations, not do both in the same die — the tapered portion squeezes the case mouth in. But it crimps it in as a cone, it constricts it evenly, and it keeps the case mouth pointed forward, not turning it inward — as the roll crimp does.
You want to do enough but not too much. If you crimp too little, the extra drag of the un-crimped mouth will cause feeding problems. If you crimp too much, you'll get a round that won't chamber or won't shoot accurately.
I have a sample that was over-crimped into a jacketed .45 ACP 230-grain roundnose, and when the case couldn't be crimped any more (the hard bullet stopped inward progress) the case then buckled. To no one's surprise, the reloader complained of unreliable function in his pistol. An over-crimped lead or plated bullet tells you it is unhappy by hurling patterns, not groups.
For taper crimp, practical shooting reloaders have been doing it long enough to have hard measurements. Take the bullet diameter, plus twice the case wall thickness and then subtract .004 inch. As a general rule, and rough estimate, you can count on a handgun case wall being right around .0001-inch thick. So for the .45 ACP, take a bullet of .452 inch, add .010 inch twice for a total of .472 inch. Then subtract .004 from that .472 and you get a goal crimp of .468 inch. Apply the same arithmetic for other calibers. A .40 or 10mm would be .400 plus .020 minus .004, which leaves us at .416.
Take your caliper and measure the diameter of a loaded round right at the case mouth. The number you get should match the calculation. (And while it's a less precise telltale sign, if you rub your fingertip down a case and feel a right-angle edge around the case mouth, the round is insufficiently crimped.)
Since many pistol bullets lack a crimp groove or cannelure, you'll be taper-crimping into the bullet itself. If you taper-crimp a revolver cartridge (yes, it works; I do it for almost all my revolver rounds) you have it easier. Since you'll have a crimp groove or cannelure in your revolver bullet, you can start with the calculation but crimp into the groove. If you want, crimp a bit more, until the edge of the case mouth contacts the bottom of the crimp groove or cannelure.
Now, not all bullets, nor all cartridges, are happy with this measurement method. If you find accuracy goes all to hell with the calculated crimp, you will have to experiment.
I have found the 9mm to be particularly touchy in this regard, especially with plated bullets. Minor variations in your particular set of dies, your press and your handguns (not to mention the brand of bullets you are using) may require small adjustments in order to produce the best results. But the above method will get you in the ballpark.
When you are setting up your press, it is best to start with a vanilla-plain load, with known performance, and load only a few rounds in each of two or three taper-crimp settings. Record where the die and lockdown rings are for each, and then check your new ammo on target.
Revolvers are spectacularly forgiving of casual taper crimp into the crimp groove, until you get up to the hottest magnum. And then they might get touchy.
Okay, what don't we use crimp for? Bullet security, a job best handled via neck tension. While the crimp does a small part to control set-back and bullet pull, the main work in that area is done by the neck tension.
You control these factors by the diameter of the neck expander in your die set. A neck that is too large to control set-back or bullet pull cannot be corrected with crimp, no matter how ferociously you crimp the case. All you do is hasten the onset of neck splits.
As an aside, you have correct neck tension when you can see the "Coke bottle" effect on your case, once loaded. If the case tapers inwards beneath the base of the bullet, like the bottom half of an old-school Coke bottle, then you have a good amount.
One good crimp/neck-tension check to perform is to load a couple dozen rounds, measure the overall length of one round, mark it and place that one at the bottom of a magazine. Shoot the magazine, except for the last round. Eject it and measure it. Has it become shorter? If not, good. Load it at the bottom of another magazine, shoot all but the marked round, eject and measure it. If it's still the same length and accuracy is good, you're done.
If it shortens due to recoil, start over with greater neck tension. Load another 10 to 12 rounds and try again. It can take a bit of fussing, but the results are worth it.