By Duane Thomas
Over-crimping, i.e. applying more crimp to the case mouth than is necessary or desirable, will deform even jacketed or hard cast lead bullets, degrading accuracy. With plated bullets, given the fact they have an extremely soft lead core, that’s even more likely to happen, Bear in mind the plating’s relative thinness compared to a true bullet jacket, therefore its fragility. We need to respect that. Over-crimp, and it is quite possible for the sharp case mouth to cut through the plating, down into soft lead, all the way around the circumference of the bullet.
Even if it doesn’t cut in quite that far, we can still weaken the plating to the point we wind up stripping the rear of the bullet plating (the “rear cup,” as I call it) off the bullet, in the bore. If that happens in the bore, it could act as a bore obstruction, bulging the barrel – or worse. However, if the rear cup strips in the chamber when the bullet hits the rifling after firing, you can actually get a failure to feed/failure to go into battery as your gun tries to stuff a loaded round into a chamber blocked at its front with a stripped rear cup.
We want our crimp to simply straighten out case flare without cutting into the bullet plating, period. Typical advice on how to set crimp goes like this: “Take the bullet diameter, measure the width of the casing wall at the mouth with dial calipers, double that number and add it to the bullet diameter.” This gives us a number to which, theoretically, we’re supposed to set the case mouth diameter of loaded rounds. Then we measure the diameter of a loaded round, right at the mouth, right at the crimp, to make sure we’ve done that.
There are multiple problems with this approach. For one thing, it ignores the fact that, in 9mm especially, case wall thickness varies immensely between makers. “No problem,” our here’s-the-magic-number fans say, “just sort casings and use all the same brand.” Yeah. And every time I switch to a different brand of casing, I’m supposed to measure the case walls, and mess with my crimp die, and reset the crimp diameter of loaded rounds, right? That’s a lot of tedious work. Thank you, no.
Also, if you can honestly hold a round of loaded ammo in one hand, and a set of dial calipers in the other, so steadily in relationship to each other you can get the jaws of the caliper riiiiiiight on the case mouth, at the crimp, and hold it there long enough to measure diameter, all I can say is you obviously have better hand/eye coordination than I do.
I needed a way to set the crimp that would (a) simply remove flare but not cut into plated bullets, (b) work across multiple brands of casings, because I don’t sort casings, I’m a total range vacuum and use a lot of “pick up brass.” What I eventually came up with is this: I use a case gage to set crimp.
A case gage is a round sided, flat topped and bottomed piece of metal, usually stainless steel, that’s had drilled into it, longitudinally, a hole equivalent to an extremely tight match chamber for a particular cartridge. If a loaded round will fit into the gage, it’ll fit into a real gun’s chamber. In this case (pun intended) however, we can take advantage of that fact and use the gage to set our autopistol taper crimp.
To start with, turn a casing backward and insert its rim into the gage, make sure the case rim will actually fit into the gage. Again, in 9mm, diameter of rims varies hugely. It’s not at all uncommon to have loaded rounds, even those that otherwise fit into the gage, bottom out on the rim and fail to fully seat. If a loaded round will not go into the gage when we’re using it to set crimp, we want to know that’s being caused by the case mouth, not the rim.
Back out the crimp die on your loading machine to the point it’s not really doing anything. Insert your “I checked and its rim will fit into the gage” casing into the machine, resize it, flare the case mouth (you can even drop a powder charge, and when you’ve completed this process you’ll have a fully reloaded, fireable cartridge), insert a bullet, and seat it. Then at the crimp station begin incrementally, minutely tightening the die. Between every tiny, partial turn, remove the cartridge from the machine and try to insert it into the gage. When it fails the gage, tighten down the crimp die a micro-turn, stick the cartridge back into the machine, crank the handle, then remove the round from the machine, try it again in the gage.
This is a very exacting thing. As you begin applying more and more crimp, the cartridge begins fitting into the gage further and further. Eventually it will begin seating all the way but won’t fall free when you upend the gage, the case mouth is still a bit too big. Keep applying little micro-turns, until the cartridge will both drop in smoothly and easily into the crimp die AND fall out of the gage when you turn it upside down.
Congratulations, you have just completely removed the flare from the case mouth, however you’ve done that to the absolute minimum necessary, without cutting into your plated bullets even a bit, and this will be true no matter the wall thickness of any casing you use.
A final note: Depending on the gun into which you’re going to be feeding this ammunition, it might work better to use the barrel in that gun as your case gage rather than a dedicated, actual case gage. But that’s a topic for a different article.
