9mm crimp

[Jack's My dog]

I know I have asked questions before about crimp, but this the most confusing and ambiguous (to me) part of the cartridge assembly process. I have been doing some more reading and I just wanted to make sure I am understanding. I know on Mag. rounds the crimp is there to prevent bullet back out, which can lock up revolvers. But if I am reading correctly, for the 9mm the crimp is really only there to remove the flare to make sure the round chambers correctly, and the bigger concern is over crimping which can lead to head spacing problems.

So my question is, if I plunk test my rounds into my chamber, and they drop in and fall out freely, without having been crimped and the gun functions as desired, do I still need to crimp the casing? It seems to me the answer would be no, but I want to make I am not missing something.

As always your guys shared knowledge is appreciated.

[67camaro]

Just a possibility if they are passing the thunk test without crimping you may not be flaring enough and may be shearing the jacket/plating off your bullets during the seating process. I would pull a bullet and make sure that isn't happening. Also make sure you are actually not crimping with your seating die. I can't remember but I think one of the die companys seating dies sizes the case again in the seating process I don't remember if LEE claimed that or not.

[Jack's My dog]

Just a possibility if they are passing the thunk test without crimping you may not be flaring enough and may be shearing the jacket/plating off your bullets during the seating process. I would pull a bullet and make sure that isn't happening. Also make sure you are actually not crimping with your seating die. I can't remember but I think one of the die companys seating dies sizes the case again in the seating process I don't remember if LEE claimed that or not.

Have had to pull a couple on the current settings and never noticed any issues, probably wouldn't hurt to pull another one just to check. I will take another look at my seating die set-up, but I am pretty sure I backed it out enough to make sure it isn't crimping.

[andrewP]

Over or under crimping *can* affect accuracy. If you're not seeing damage to the jacket or plating when you pull rounds, you're probably not overdoing it. I'd just take your calipers to some rounds you've loaded and make sure the case mouths are between .376 and .378. If you're in that range, I'd bet you're crimping during the seating step, and you managed to get your seater die set up correctly to seat and crimp in one step. (Yes, many people recommend against doing that, but as far as I understand, that's mainly because it's supposed to be hard to get it set properly.)

[crbutler]

Crimping is to keep the bullet in the same place. While sometimes having a good crimp ensures even powder ignition, that should not be the case with a 9mm.

If the case is properly resized, the neck tension should hold a 9mm bullet in place. (should not always...)

In a 9 mm you should be using a taper crimp and the best way to measure it is to seat and crimp, then pull the bullet and measure the difference at where it crimped. Ideally you want 3-5 thousandths of crimp with a jacketed bullet in an autoloader. However, every gun and load like things a little different. You can experiment with this.

If you are shooting cast lead, they often have a crimp groove and this method doesn't work. There I just crimp until I can see that the case is crimped in the groove- but you need to clean the die frequently as the bullet lube will build up.

If you want to check the neck tension, take a completed round, and press the bullet end into the table as hard as you can bare handed with your fingers. Re measure OAL, if it changes more than 1-2 thousandths, your neck tension is not good enough and you need to crimp more.

Plated bullets are the real fly in the ointment. You need to bell the case enough to seat them without damaging the plating, and crimp them enough to keep them from shortening on you. the accuracy of your loads will tell you how good you are doing here- but don't skip checking for bullet movement as that is what causes the case volume to shrink and can raise pressure dramatically.

Revolver bullets usually require a roll crimp and need a crimping groove to be done- this will look like the lead bullet comment above.

[Jack's My dog]

I just measured my flare on an empty case, it measured .377- this amount allows me to seat the bullets finger "tight" and consistently "plumb". Any less flare allows the bullet to seat out of "plumb" when run through the press, and bulges the side of the case. Case mouth with a seated bullet is at .378 and required a couple hard wraps with the bullet puller to dislodge. No obvious shearing or marring noted to the plating. Casing mouth diameter after pulling the bullet-.376

[Jack's My dog]

I just checked my test bullet I had sitting at my desk, I leaned pretty good on it, and didn't measure any noticeable movement in the bullet. I will spot check a few more.

[crbutler]

You need to measure the bullet diameter, not the case after pulling.

You should see a small line on the bullet jacket. You measure just to the base of that.

[Jack's My dog]

You need to measure the bullet diameter, not the case after pulling.

You should see a small line on the bullet jacket. You measure just to the base of that.

I measured the case after pulling the bullet just out of curiosity, noting that the the cases return after the bullet had been pulled.

[Seismic Sam]

With autoloaders part of the equation is keeping the bullet from setting back into the case when it whacks the feed ramp and chambers. Bullet setback in 40 S&W is a known cause of Kabooms in early gen Glock chambers. 9mm is generally not a problem, provided you don't seat the bullet too deep. One of the oddities of the 9mm bullet is that the ogive (curve of the nose) extends back over HALF the length of the bullet in RN bullets, so if you seat too deep and crimp you are making the ID of the case mouth SMALLER than .355". That may lead to increased pressures, so pay attention to OAL's for each bullet and do NOT go UNDER the OAL. If you want to check, load up a full mag OUTSIDE, point the gun at the ground, and continuously rack the slide full length and let go to cycle the bullets, and then check the OAL's again. As far as case mouth belling, you want to bell it just enough so that the first 20 mils of the bullet stick in the case mouth after being pressed in with the finger.

If you want to measure the crimp on plated or jacketed bullets, crimp a few, and then pull the bullets and measure how deep the crimp mark is on the bullet. For a .355" bullet, if the OD of the crimp is .353", then the crimp is 2 mils, or 1 mil per side. Don't confuse those two ways of measuring it!!