Crimping 9mm??

[MegaPlinker]

I hope to start reloading soon and will be starting out with 9mm.
I have heard some people say that crimping helps the accuracy of their 9mm. Others have said it is not needed since it uses a taper crimp.
When I buy my die set is it worth getting the factory crimp dies?
Thanks.

 

[Lugnut]

I'm no expert but 9mm luger uses a taper crimp (as opposed to a roll crimp used on many revolver cartridges). No idea about accurracy pluses but I do it. Too much crimp is bad and can cause excessive pressures.

 

[Finalygotabeltfed]

The 9mm luger cartridge headspaces on the case mouth. Never roll crimp a 9mm as it will effect function. Taper crimping is ok.

Excessive crimp in any cartridge will increase pressures and you should be aware of this if you are loading near the top end of a particular load as it may place the load in an unsafe pressure range for the firearm being used.

A Dillon taper crimp die or a Lee Factory Crimp die will work just fine.

Don't forget to gage your ammo.

 

[RKG]

1. Take heed of the note above about headspacing on straightwall auto cartridges; use only a taper crimp and not so much as to eliminate the distinct shoulder at the mouth of the case.

2. As for how crimping affects accuracy:

A) As everyone knows, the primary contributor to cartridge accuracy is consistency of velocity.

B) Chamber pressure relates more or less directly to bullet velocity.

C) As noted, the force required to start the bullet moving (and, thereby, increasing the combustion chamber volume) has some effect on early chamber pressure.

D) That force is the sum of bullet inertia plus natural clamping force of the case mouth itself (without any crimp) plus the additional clamping force of the crimp.

E) The inertia of all bullets of the same mass (weight) will be the same.

F) Particularly when shooting brass that has been reloaded a few times or mixed brass, the amount of natural mouth clamping force will vary widely.

G) Adding a consistent additional clamping force from crimping tends mathematically to reduce the fractional variability of the sum of case mouth clamping plus crimp clamping (since we are adding a fixed value to a variable one), and thus in theory tends to make more accurate rounds.

The fly in this theoretical ointment lies in case dimensional variability, particular case wall thickness in the mouth area. Variability in case mouth wall thickness translates directly into variability in crimp clamping force, since clamping force comes from the effecting inside diameter of the mouth will the crimp die affects the case mouth outside diameter. Thus, the condition required in premise G), above -- namely, consistent crimp force -- is impossible to achieve.

The bottom line is that no or too little crimp will affect round reliability, as will too much crimp. Whether there is an accuracy contribution of various values between these extremes is a matter of uncontrollable factors and, therefore, isn't worth worrying about.

 

[EddieCoyle]

If you'll be reloading on a progressive press, then by all means get a factory crimp die; I recommend the Lee FCD. You'll want it not so much for the actual crimp (although the Lee FCD gives very consistent results), but to allow you to separate the bullet seating and crimping operations.

Combination seating and crimping dies can be difficult to adjust. Also, with these dies, the crimp is being appiled while the bullet is being seated - in other words, while the bullet is still moving. The Lee FCD resizes the finished case after crimping; so if you over-crimp, the die will remove the bulge that occurs just below the top of the brass. I have yet to have a round fail to chamber after it came out of a Lee FCD.

If you're reloading in a single stage press, you can simply use the combination die first to set the bullets, then after backing out the seating plug, to separately crimp.

G) Adding a consistent additional clamping force from crimping tends mathematically to reduce the fractional variability of the sum of case mouth clamping plus crimp clamping (since we are adding a fixed value to a variable one), and thus in theory tends to make more accurate rounds.

There's no such thing as "crimp clamping" with a taper crimp. A taper crimp does not help the case to hold the bullet. In fact, a stronger taper crimp will bulge the case just below the top edge, causing that part of the brass to not be in contact with the bullet, thus reducing surface area, friction, and holding power.

All a taper crimp does (and should ever do) is remove the bell. The word "crimp" is misleading in this case. A more accurate term would be "flattening".

 

[RKG]

. . . . There's no such thing as "crimp clamping" with a taper crimp. A taper crimp does not help the case to hold the bullet. In fact, a stronger taper crimp will bulge the case just below the top edge, causing that part of the brass to not be in contact with the bullet, thus reducing surface area, friction, and holding power.

All a taper crimp does (and should ever do) is remove the bell. The word "crimp" is misleading in this case. A more accurate term would be "flattening".

I'd like to see the materials that purport to demonstrate this proposition.

It is true that the amount of crimp clamping force that can be applied by a true taper crimp prior to the point of case deformation is less than the amount of crimp clamping force that can be obtained via a roll crimp, the notion that passing a loaded straight wall round through a taper crimp die (set otherwise than for zero crimp) results in zero crimp clamping force defies the laws of physics.

 

[EddieCoyle]

I'd like to see the materials that purport to demonstrate this proposition.

It is true that the amount of crimp clamping force that can be applied by a true taper crimp prior to the point of case deformation is less than the amount of crimp clamping force that can be obtained via a roll crimp, the notion that passing a loaded straight wall round through a taper crimp die (set otherwise than for zero crimp) results in zero crimp clamping force defies the laws of physics.

I meant excessive crimp, not a die set "otherwise than for zero crimp". A mistake that many newbies make when a bullet is not held tightly is to increase the taper crimp in an effort to improve the case's ability to hold the bullet. It's not the way to fix the problem.

Many die sets have a crimping die that taper crimps to a point, then roll crimps when adjusted further down. If you adjust this die down to "get a stronger crimp", it will begin to roll crimp. You'll deform the brass when the case mouth hits the lip inside the die that performs the roll crimp. Since the bullet has no cannelure, there's no place for the case mouth to roll into and the brass is pushed down. The brass immediately below the crimp will bulge out, away from the bullet. In this case, excessive crimp results in a net crimping force of less than zero when compared to an uncrimped round.

When using a true taper crimp die (where the inside of the die is only tapered, and the mouth of the case does not bottom out on a lip), excessive crimping force does not increase the "holding power" of the case because the metals involved, lead and brass, have different moduli of elasticity. (I apologize in advance if this is basic stuff - I don't mean to be pedantic). Try this, take a strip of lead and a strip of brass and put the ends in a vise. Take the ends that are sticking out and bend them the same amount. If you bend them a tiny bit, they'll spring back to their original positions. Bend them a little more and the brass will spring back all the way, but the lead will only partially return. You've exceeded to modulus of elasticity of the lead. If you over crimp, you'll quickly exceed the modulus of elasticity of the lead and the brass will "sping back" some amount when the clamping pressure is removed.

In this case, excessive crimp results in a net crimping force of zero when compared to an uncrimped round. The top of the case was not in contact with the bullet before crimping, and will not be in contact with the bullet after crimping.

My point is, if the case is not sufficiently holding the bullet before you taper crimp, over-crimping the round will not improve the situation. If you are able to apply so much over crimp that the case somehow grabs the bullet enough to hold it (usually by deforming the jacket) you'll end up with cartridges that are inaccurate, inconsistent, and maybe even unsafe.

 

[RKG]

Well, you're on to my point and possibly beyond my qualifications, but:

a) you have to evaluate the elasticity of the lead under compression (i.e., the round section of the bullet being compressed on all sides) vs. bending (made more complicated by the fact that we are probably talking about a jacketed slug, possibly FMJ); and

b) in order to assert zero crimp clamping force, you have to assume that the brass mouth of the case, after it is withdrawn from the crimp die, rebounds entirely to its pre-crimp diameter. That is not my observation.

Minutiae aside, I think we can agree that (a) that the marginal crimp clamping force from a properly applied taper crimp is very low and (b) that added to the larger equation of "accuracy" (read: consistency of velocity) affecting factors, can be ignored.

 

[EddieCoyle]

b) in order to assert zero crimp clamping force, you have to assume that the brass mouth of the case, after it is withdrawn from the crimp die, rebounds entirely to its pre-crimp diameter.

Not true. It doesn't have to spring all the way back. All it has to do is spring back more than the slug. Whether it's a thousandth of an inch or a thousand miles away - if it's not touching it, it ain't applying any friction.

You are right though - we pretty much agree, and are now splitting hairs.

Cheers!

 

[RKG]

If only we could do that with 9mm's as effectively as we do it with words?

 

[EddieCoyle]

If only we could do that with 9mm's as effectively as we do it with words?

Hijack time: Why would you reload 9mm anyway? I'm a fanatical reloader. I'd rather shoot my own reloads than anything from the factory, but it is more expensive to reload 9mm than it is to buy it.
I reload so I can buy guns in calibers like 10mm, .500 S&W Mag., .44 SPL, and even .45 and .32 ACP, but 9mm? Why bother?

 

[Lugnut]

Hijack time: Why would you reload 9mm anyway? I'm a fanatical reloader. I'd rather shoot my own reloads than anything from the factory, but it is more expensive to reload 9mm than it is to buy it.
I reload so I can buy guns in calibers like 10mm, .500 S&W Mag., .44 SPL, and even .45 and .32 ACP, but 9mm? Why bother?

Per 1000-
Powder $12
Primers $22
Good FMJ bullets $50
Used cases $0
Total $84

Most places are selling decent 9mm rounds for $140 +/-. Not including labor its a fair savings. Plus reloading is fun.... you know that.