More crimp = more accurate?

[JKilbreth]

I just bought a Dillon RL550B press setup for 9mm. I previously reloaded 9mm on a Lee single stage press. On the Lee press, I used very little crimp, as I noticed that the majority (maybe all, not sure) of factory ammo I buy has little to no crimp. I fired those rounds with acceptable accuracy. I didn't even really have a problem with it until I began reloading the same 9mm on my Dillon with [what I consider to be] heavy crimp. That's when I noticed that my shots became more accurate! My shot groups at 25' with the Lee reloads were about the size of a salad plate (no judging, please), but the reloads I shot off the Dillon grouped tighter than a saucer plate at the same distance.

I tried my best to eliminate the variables from the experiment: Used the same powder drop, same bullet, same case, same pistol, even the same indoor range. Same results.

My question is this: Does more crimp equal more accuracy? I've never thought of crimp affecting accuracy that much. I always thought of crimp as having the primary function of delaying the forward movement of the bullet (ideally to allow for more powder to burn before the bullet leaves the barrel). But is accuracy a common added bonus?

Thanks in advance!

 

[steve4102]

Do not over crimp. Only apply enough crimp to remove the flare plus maybe a thou more.

 

[Mike / Tx]

Your question is a valid one, and one that I have tried to pass on to others who are starting out loading handgun ammo.

Most who I refer this to are loading for revolvers but it also applies to semi auto handguns as well.

The case tension is more or less the best gauge to go by when working up loads, however the crimp will also play a significant role as well. If you have a decent set of dies the sizer should bring the case down enough to put sufficient tension on the bullet so that all the crimp actually does is simply hold the bullet in place under recoil or when chambering from the magazine. Sometimes however even when the die brings the outside down the expander will open it right back up to bigger than it needs to be. This could also depend upon the size of the bullets being loaded. If they are lead or jacketed as the two are different sizes, as can be two different brands of jacketed. In such a case the expander ball can be polished down just a touch to increase the tension, but this is usually in rare circumstances with a good name brand set of dies.

It is usually when folks read things like "use a heavy crimp" that things head south. While I will fully admit that using a heavy crimp can be important, it can also destroy accuracy as well. In most cases, if everything else is as it should be, all that is needed is enough of a crimp to remove the bell on the case mouth for automatic ammo, or to simply roll the edge of the lip into the cannalure on a revolver round. I have seen the crimp easily make differences of up to 6" at 25-50yds with everything else being the same. In your situation you probably also setup your Dillon with their die set which could also effect the groups your seeing as well.

There are, as mentioned exceptions, but I have found that it is usually better to start off light and work into it rather than to start off swedging the jacket or sizes of the lead bullets down to begin with. Lead is slicker than copper so most lead bullets might require a touch more input from the crimp, however even my hot loaded rounds for my 454 are still simply rolled into the crimp groove rather than pinched into it. While the picture isn't the best I have taken, hopefully you can still see what I am talking about in this picture.

If you look closely at the loaded rounds you will see the profile of the bullet within the side of the case neck. You can clearly see the lube groove and the slight appearance of the gas check. This is what I mean by case tension. Also you can see that the crimp is simply rolled into the crimp groove, and not pinched down so much that the rim of the case is distorted.

With a loaded semi auto round you should only crimp down enough to remove what little flare had to be added to the lip of the case in order to seat the bullet. The case rim should in most cases be almost parallel along the side of the bullet. Most semi auto rounds headspace on the lip of the cases is why. That said your chamber dimensions might also dictate what you can or cannot get away with. This is why you see the "plunk test" so often suggested.

So as mentioned above, good question, and hope I helped answer it.

 

[Hammerhead]

You might have too much flare/expansion of the case mouth and the crimp is 'repairing' the over expansion and lack of bullet tension.

In 9mm my best ammo is made with no crimp at all. (it tests very well at 25 yards) I ensure I'm using just enough flare to keep the case mouth from scraping the bullet. With jacketed bullets I barely touch the cases with the expander and let the bullet do the expanding. Then just a kiss with a taper crimp to remove any trace of flare. No inward crimp at all.

 

[JKilbreth]

Thanks for the input!

Not really sure why I thought this, but wouldn't cartridges feed and chamber better if the transition from the bullet head to the cartridge case over the feed ramp is a smooth one (i.e. crimp makes the cartridge smoother...)?

I'll make several cases both ways and see, but I was just curious.

Also, I'll try the "plunk" test.

If the info helps any, I'm using:
115gr Jacketed round nose bullets (not sure if copper or other)
3.4 grains of Alliant Green Dot smokeless shotshell powder
Mixed headstamp found brass
CCI Small Pistol primers
Firing out of a Gen4 Glock 17

Thanks!

 

[Hammerhead]

Not really sure why I thought this, but wouldn't cartridges feed and chamber better if the transition from the bullet head to the cartridge case over the feed ramp is a smooth one (i.e. crimp makes the cartridge smoother...)?

Never seems to be an issue with 9mm. With 1911s in .45 auto it can be a factor.
I've run several thousand 9mm practice rounds where I did not flare or crimp at all. I resize/reprime, charge, then seat the jacketed bullet without even using the expander die or crimp die (no crimp in my 9mm comp. seater die). Never an problem.
Most modern 9mm pistols are very tolerant of bullet shape and case mouth shape. I never could make my my Glocks or HK's choke on my reloads unless they didn't have enough powder to cycle the slide.

 

[g.willikers]

Or maybe the improvement in accuracy is more due to the quality of the press.
Like bullet seated straighter in the case, for example.

 

[buck460XVR]

There really is no such thing as "crimp" in autoloader rounds. It is a misnomer. You are only removing the flare. What holds the bullet in is neck tension and this may be greater in your handloaded rounds than in factory ammo. The powder/bullet combo you use in your handloaded ammo may just be more accurate than the factory load also.

 

[Hammerhead]

I have a box of factory Winchester USA .40 180 gr. JHP that has a substantial crimp. The jacketed bullet is obviously deformed and I measure .409" at the case mouth, it might be a smidgen smaller, but my calipers slip off. The case over the bullet measures .420", so there's more than .010" of actual crimp.
Of course that's rare to see on factory rounds. I think Winchester was overly concerned about these rounds being re-chambered over and over.

Most factory auto pistol ammo is not crimped however, and doesn't need to be, just like our handloads.

 

[Hammerhead]

Or maybe the improvement in accuracy is more due to the quality of the press.
Like bullet seated straighter in the case, for example.

That could be a big factor as well.

 

[dahermit]

Or maybe the improvement in accuracy is more due to the quality of the press. Like bullet seated straighter in the case, for example.

Am I wrong in assuming that you meant "...due to the quality of the dies.."? I do believe that in most cases, the dies control the how straight the bullet is seated, ie., dies that support or do not support the bullet when seated.

 

[Slamfire]

You are talking about a handgun. All you have to do is hold within two inches at 25 yards to shoot a perfect score on one bullseye target.

Handguns are hard to shoot accurately so I really doubt a heavy crimp will take that much away.

Heavy crimping ruins the accuracy of rifle bullets. Heavy crimping deforms the soft lead core and changes the center of gravity. This affects accuracy the further you go out.

But for a pistol, I consider pistols spitting distance weapons so in my opinion, you want maximum function.

And that is basically taking out the flare.

 

[Hammerhead]

I replaced my Lee 9mm seating die because I could see the seater stem tilt sideways a little each time it started seating a bullet. No way that's good. I bought a Redding competition seating die and accuracy did improved overnight. The competition seater also allows you flare less or not at all, and that allows you to crimp less. I think the less you work the case mouth back and forth the better. Better for accuracy and better for brass life.
Since switching to the new seater I've had very few plated or lead bullets get skinned by the case mouth, another good sign of proper bullet alignment.