Measuring the Crimp

Measuring the crimp is arguably one of the most difficult measurements to take when reloading ammunition.

The crimp is where the brass shell is squished into the sides of the bullet. It occurs at the very end of the shell, at the right-most end of the brass in the pictures below.

The crimped area is circled in red in the magnified (second) image. Notice that it slopes inward. There is no plateau on which to set the jaws of your calipers.

Most crimps, regardless of whether they are the tapered or rolled variety, will be either conical or spherical when viewed from the side. That means you can’t measure the crimp just any old place. The amount of crimp grows tighter and tighter toward the mouth of the shell.

These pictures show a tapered crimp; it has a conical shape. It was made by a Lee “Factory Crimp” die on a 45 ACP cartridge.

The 45 ACP cartridge requires a tapered crimp because that’s how the shell indexes on the chamber – on the mouth of the shell. Roll crimps are used when the indexing is done off the base, or shoulder if it has one, and it bites into the bullet. A tapered crimp presses (hard) onto the sides but doesn’t cut in.

The roll crimp, on the other hand, bites deeper into the bullet. It literally “rolls over” the end of the shell so it presses inward around or into the projectile.

The type of crimp, rolled or tapered, is made by the shape of the crimping die which contacts the shell at the extreme end of the mouth. In the picture above, the crimp only affects the last 1/32" of the shell. A straight edge held against the side of the shell with a bright light on the other side will help.

In the pictures above, notice how the gap between the straight edge and the brass is sloped for the last fraction of an inch. It is a flat slope inward and, because it goes all the way around, it has a conical shape. But since the whole cone isn’t there, it would be better to say the mouth of the brass is shaped in a conical section.

But the key feature as far as taking a measurement is the fact that the brass slopes. The diameter of the crimp, of that conical section, depends on where you measure it.

When the crimp is supposed to be 0.469" as in my 45 ACP reloads with the 200 grain LSWCs, that measurement is only at the very end of the conical shape. This is not an easy point to consistently place your calipers from one round to the next.

Here’s what you want: the jaws of the calipers need to be in contact with the brass at its very end. If the calipers are placed farther down the shell away from the mouth, you won’t be measuring the narrowest part of the crimp.

In the next picture you can see the thinnest part of the calipers jaws being used and, at least on the left, the face of that jaw is half-on and half-off the brass. It is right at the edge of the brass. Even so, the caliper’s jaws look huge compared to what they’re attempting to measure.

Getting the calipers into that exact spot is hard, but that’s only half the story. You also need to use only the thinnest part of the caliper’s jaws, the part out near the end.

If your eyesight is extremely good, you may be able to do this just by looking.

I can’t.

So, I use a different sense.

I do it by touch.

I place the calipers close to the end of the brass and then I move one jaw up and down feeling for the edge of the brass and, once found, I “plant” it on the brass. I then do the same with the other jaw and, at the same time, snug up the jaws. When I feel that both jaws are “on” the brass, I’m take the reading.

Now, I usually know what numbers to expect. If I measure in the right spot, I ought to see 0.469" ± 0.002", that is anything from 0.467" to 0.471". If the measurement is outside of that range, I’ll take the measurement again and pay more attention to placing both jaws on the brass, not on the bullet.

But if I get the wrong number, then I want to get several of the same reading before accepting it.

It is, after all, easy to have one jaw on the bullet and the other on the brass and get a reading smaller than 0.467". And if the calipers are cock-eyed, then I can get a reading that’s too big.

Both jaws have to be in the right place to measure the real crimp.

So I feel my way onto the end of the brass and, if I get the number I expect, I assume I’ve taken the reading in the right place.

If I get something different, then I take the calipers off, open the jaws and start over. If I get another “off” reading, then something may be wrong with that round. Or if I get the reading I was hoping for, then I’ll do it one more time to be sure.

I’ve toyed with the idea of creating some sort of jig that would help keep the calipers  square with the round and in the correct place at the edge of the brass but, frankly, the “by feel” method seems to work quite well. Once you get the hang of it, it’s quick and reliable.

Why bother with yet another tool when the “by feel” approach works?

It just needs a little practice. Pay attention to what you’re doing and you’ll get it right.

After all, your sense of touch is probably very good. Bullseye shooters need a sensitive touch on the trigger to make that shot break cleanly. You’re almost certainly used to paying attention to what you feel.

An oddity I reported here at this link while chasing a different issue is that 45 ACP brass typically gets shorter with repeated firings. Because the crimp operation indexes to the base of the brass, as the brass gets shorter, less and less crimp will be applied. Exactly how that affects the accuracy of the round is unclear, but my gut says it’s important at the higher levels of competition.

For the momenbt, I will continue reloading brass until it splits or otherwise craps out, and I will continue to use the “by feel” method to check the crimp.

Touchy, feely. Touchy, feely!