Understanding Crimp

[Ben Diss]

I'm trying to understand how crimp effects a .45 ACP load. I understand the difference between taper crimp and roll crimp, but I don't understand how crimp effects peak pressure, muzzle velocity, etc. Here's what I've read:

- try not to think of crimp on .45 ACP and anything other than removing the bell

- crimp just enough so that the diameter is consistent through the whole length of the case

- don't crimp enough so that there's a dent in the bullet (if you remove it and examine it)

- crimp .45 ACP to .469 for lead and .470 for jacketed bullets

Clearly these instructions are contradictory.

I've been crimping MG 200 JFP to .471-.472 and today tried .469-.470 (leaving a small dent in a removed bullet) and found that for lighter loads, this increased muzzle velocity 45 fps and for heavier loads deceased MV by 30 fps.

What gives?

Edited March 30, 2011 by Ben Diss

[lugnut]

I agree with the 1st 2 lines in your quote. Sometimes- even with a taper crimp though- you will mark the bullet.. not a big deal. I do this all the time even with plated bullet.

Honestly though I'm surprised at that much velocity difference... the throat tension is what really holds the bullet anyway.

[Erik S.]

This is MY opinion only and is not intended to overrule someone with concrete scientific evidence of the effects of bullet crimp.

I think of crimp as removing the bell, also (as stated in your quote). I crimp so that each round chamber checks and sort of rounds the sharp edge around the outside case neck and so each round feeds reliably. Removing that sharp edge accomplishes that for me. I don't think of it as cutting into the bullet to hold it there. Unless you're having bullet seback issues, this approach might work for you. Because case length can vary drastically, the crimp can also vary drastically since the die crimps based on contact with the case rim.

Because of the inherant and uncontrollabe inconsistencies, I rely almost entirely on "neck tension". The .45 doesn't have a "neck" so that term isn't entirely accurate, but you catch my drift. I don't know anyone who trims each pistol case. That is the only way to achieve a consistent crimp. .002" or .003" case variation will not significantly change your coefficient of friction between the case and bullet but those few thousanths could be the difference between crimp and no crimp if you have your die set up to do so. I would much rather round that little edge off and call it good.

I tested this in an experiment where I drilled holes in bullets sideways and seated them...tied fishing line through the hole and (using a shell collet) pulled each bullet measuring the force required with LabView and the required sensors.

The cases were of varying length and crimped the same. Other cases also varied in length by the same measurements and did not have crimp applied. There was a spread in each test but far more drastic with the crimped cases. The shorter cases had almost no crimp while long ones had quite a bit. Seating/crimping was done with a lee seating die.

Moral of the story, use powder charge to control pressure/velocity and seating depth to ensure reliable feeding.

[Ben Diss]

I agree with the 1st 2 lines in your quote. Sometimes- even with a taper crimp though- you will mark the bullet.. not a big deal. I do this all the time even with plated bullet.

Honestly though I'm surprised at that much velocity difference... the throat tension is what really holds the bullet anyway.

It also interesting how the lighter loads increased speed and the heavier load decreased.

This is MY opinion only and is not intended to overrule someone with concrete scientific evidence of the effects of bullet crimp.

I think of crimp as removing the bell, also (as stated in your quote). I crimp so that each round chamber checks and sort of rounds the sharp edge around the outside case neck and so each round feeds reliably. Removing that sharp edge accomplishes that for me. I don't think of it as cutting into the bullet to hold it there. Unless you're having bullet seback issues, this approach might work for you. Because case length can vary drastically, the crimp can also vary drastically since the die crimps based on contact with the case rim.

Because of the inherant and uncontrollabe inconsistencies, I rely almost entirely on "neck tension". The .45 doesn't have a "neck" so that term isn't entirely accurate, but you catch my drift. I don't know anyone who trims each pistol case. That is the only way to achieve a consistent crimp. .002" or .003" case variation will not significantly change your coefficient of friction between the case and bullet but those few thousanths could be the difference between crimp and no crimp if you have your die set up to do so. I would much rather round that little edge off and call it good.

I tested this in an experiment where I drilled holes in bullets sideways and seated them...tied fishing line through the hole and (using a shell collet) pulled each bullet measuring the force required with LabView and the required sensors.

The cases were of varying length and crimped the same. Other cases also varied in length by the same measurements and did not have crimp applied. There was a spread in each test but far more drastic with the crimped cases. The shorter cases had almost no crimp while long ones had quite a bit. Seating/crimping was done with a lee seating die.

Moral of the story, use powder charge to control pressure/velocity and seating depth to ensure reliable feeding.

Fascinating. I hadn't considered case length, but you're absolutely right that it will effect crimp. I measured each round in my test, and found no more than .001 variance, but over several hundred there has to be more variation.

Can you share some of the results of your tests? What kind of pull forces did you measure?

[Erik S.]

Ill look in some of my old notebooks at home to see if I still have actual forces but I did the experiment back in my early college days so may have thrown it away......stupid if I did considering I am an avid reloader who is interested in this type of thing.

[pjb45]

I worked with a commercial remanufacturer for a while with my 40S&W load. I told him I wanted a LFC die because I was getting some FTF issues. This was advised to me by some very knowledgable people.

He argued with me about the need to crimp it. He went on to show me article about rifle shooters and how the crimp had an adverse effect on accuracy. Needless to say, he did use a LFC on my stuff. I made him a believer.

I put a slight taper crimp on my 9/40/45/223. Just enough to knock the hard edge off. I get great groups out of my ammo.

I had too much of a crimp on some hot 223 stuff. The pressure blew the primer out.

[Erik S.]

*snip*

I put a slight taper crimp on my 9/40/45/223. Just enough to knock the hard edge off. I get great groups out of my ammo.

*snip*

Yep, that's what you want....just enough to knock the edge off but not enough to bite the case into the bullet (lead OR copper)

[NoSteel]

I like a light crimp with my LFC die due to set back issues I encountered once when I was trying to run "no" crimp on the LRN's I was using. I see no keyholing and accuracy remains stable.

[Ben Diss]

What's an LFC die?

[dskinsler83]

Lee Factory Crimp die

[Erik S.]

I like a light crimp with my LFC die due to set back issues I encountered once when I was trying to run "no" crimp on the LRN's I was using. I see no keyholing and accuracy remains stable.

If you're seeing bullet setback, then yes, you WILL need to crimp. Nothing wrong with that, but it's just unfortunate because it forces you to trim the length of every case so that the amount of crimp is uniform. I typically switch bullets (or sizing dies) WAYYYY before I'm forced to trim cases. JMO

[Ben Diss]

Thanks for everyone's help here. I think I've got a better idea of how it works now.

[stinsonbeach]

Here's my .02 -

On my .40's & .45's:

I use the regular bullet seating die to taper-crimp (.4230 for .40; .468-.4710 for .45)

Then if I feel or see any abnormalities I run it thru my roller-crimp-die WITHOUT using the roller crimper (I have it backed all the way up) - just to straighten the case.

Any problems you may have ought to be taken care of there.

Edited April 19, 2011 by stinsonbeach