How much "difference" does crimp make?

[Prof Young]

Loaders:

How much difference does crimp make in the performance of the bullet?
I'd assume that more crimp means more pressure build-up before the bullet starts to move down the barrel.

Does this increase in pressure mean an increase in speed?

More pressure means more case expansion . . . right? Could that cause more failures to eject in a semi-auto.

So much to learn about ballistics etc.

Live well, be safe.
Prof Young

[Unclenick]

Increasing crimp increases the start pressure of the round by making the bullet take a little longer to free up and start moving. However, this all happens when the pressure is low on the rising portion of the curve. It does result in a little bit higher peak pressure and a little bit more velocity results, but it's not huge. Board member Hummer70, who used to work at Aberdeen Proving Grounds said he'd tested bullet pull on old 7.62 ammo which had been 60 lb when new, but by oxidation cold welding had got as high as 600 lb, way more than a crimp can introduce, but that still fired fine. So the pressures and velocity changes are not very significant. What is significant is the velocity consistency can improve and that indicates improved ignition consistency. With some powders, particularly the slower spherical powders, this can improve performance. For stick rifle powders it does not seem to be necessary if you clean and size your cases consistently and prime correctly.

The way to tell if crimping is helping ignition consistency for you is with a chronograph. See if the velocity SD and ES decrease as a percentage of the mean velocity. But even if they do, also check group size on paper. Getting that added consistency of velocity is no good if you are distorting a bullet to the point it no longer flies straight.

[Bart B.]

A "trick" used by several military rifle team members shooting M72 .30-06 and M118 or M852 7.62 NATO match ammo was to "uniform" the case neck's grip on the bullet. Some lots of match ammo were either old or had poor quality black asphultum sealant between the bullet and case neck. Case necks were sized to be a thousandth or so bigger than the bullet so there was room for the melted sealant to be put in then cool and hold the bullet in the case mouth. If the "grip" that sealant had a wide spread, muzzle velocity spread was also wide. Vertical shot stringing as much as 1 to near 2 MOA was added at 600 yards and as much as 3 MOA at 1000.

I used a Lyman 310 tong tool (the old 'nutcracker') with a .308 Win. seating die in it set to seat bullets back about .005" or so to break that seal on M118 and M852 ammo. Such modifications made the case neck's grip on the bullet more consistant; the lighter grip didn't seem to matter at all. Accuracy improved to what otherwise good ammo would shoot. Wasn't allowed in DCM matches as ammo had to be shot "as issued."

Arsenals originally crimped case mouths on .30-06 match ammo and the asphultum sealed bullets had a cannelure turned into them for the case mouth to be curled into. Arsenal accuracy tests in the mid 1900's proved such crimping was detrimental to accuracy so they quit crimping case mouths. Next, they quit canneluring that old match bullet used since the early 1920's. The end result was better overall average accuracy.

Sierra Bullets makes several products with cannelures intended for folks who want to crimp case mouths into them. But such bullets are tested in cases uncrimped into bullets. They proved decades ago that crimping makes bullets shoot less accurate. Sierra's objective is to test bullet quality based on how they were sold to customers. Customers could do whatever they wanted to with them in their cases.

[reynolds357]

Prof Young, in my opinion, pistol ammo should be crimped. In certain specific instances, such as a tube magazine, rifle ammo should be crimped. Other than that, the loss of accuracy is not worth the "benefits" of crimping.

[44 AMP]

How much difference does crimp make in the performance of the bullet?

It can make a lot of difference, depending on many factors, and which round, and what gun it is being fired from.

The purpose of a crimp is to ensure the round "performs" properly until the moment of firing. To keep the bullet in place during its time in the magazine and through the feeding cycle.

In some guns and calibers this is a significant concern. In others not so much, and in some, none at all.

Where it is a concern, the slight difference in accuracy between a properly crimped round and an uncrimped one is accepted to ensure proper function. In many cases where gun design needs crimped bullets the guns themselves are not capable of showing the slight difference in accuracy between crimped and non crimped ammo. (assuming the crimp is done "right". A bad crimp is often worse than none)

One might be throwing away match winning accuracy by crimping (long range rifle match), or you might have bullet jump freeze your revolver from NOT crimping. It all depends very much on the specific of what you are shooting, and what you want out of it.

Load a magnum pistol case with a slow burning powder, and no crimp, and despite a magnum primer and good neck tension on the bullet, the odds are high you will have erratic burning of the powder. That's one case where a crimp is needed.

Heavy recoiling magazine rifles are another.

Pistols, Rifles, revolvers, semi autos, bolt guns, single shots, all in multitudes of calibers, how much difference a crimp makes to any one can be completely different with another.

[Nick_C_S]

How much difference does crimp make?

About two weeks ago, I was wondering the same thing. So I used my trusty 38 Special 2.95gn Bullseye under a 148 DEWC recipe (my IDPA load) to run a test. They are taper crimped. So I took 10 rounds and "retro" roll crimped them. Off to the range I went, with chronograph in tow.

S&W M67 4" bbl; 10-round sample each:

Taper: 754 fps
Roll: 767 fps

IMO, the results are not statistically significant; especially considering these were "production" powder-dropped rounds, and not "blue print" hand-weighed rounds. Point to ponder: DEWC's fit deep in the case with a lot of bearing surface contact (if that's the correct term). So this bullet profile would likely show less difference between a medium taper and a medium-heavy roll crimp than most others.

I'm not sure how pertinent this test is to the OP's question. But I figure sharing data is always good.

I do plan on running similar tests on other bullet types.

[Hammerhead]

Crimp should have little effect in an auto pistol because you should have little to no crimp. With normal auto rounds like 9mm, .40 & .45acp, bullet tension (neck tension) should be the main gripping force on the bullet. All of my auto rounds vary between no crimp and .003" crimp. The variation comes from minor differences in case length.

Magnum revolvers rounds are another story where the powders are slower, the charges much heavier, and the cases are generally much longer by comparison. Crimp is a major factor there. Some powders may not even ignite without a good solid roll/profile crimp.

[Unclenick]

Well, I was thinking just rifle in my first post. For pistol, as others said, it's a different animal. I've never found a crimp helpful with jacketed pistol bullets. They have pretty good friction with the case without that. However, with lead bullets it is another matter. Back in the 40's and 50's and 60's, when roll crimp shoulders were what all seating dies seemed to come with, competitive .45 Auto shooters all swore that a roll crimp into their lead bullets a little bit behind the leading edge of the bullet produced best 50 yard accuracy. Mind you, the bullets were seated out far enough that the cartridge headspaced on the bullet's contact with the throat and not on the case mouth, which is why this did not introduce a pressure issue.

In the early 80's, when I experimented with headspacing on the bullet, I found that alone improved accuracy significantly. I was roll crimping then, but only gently. Later taper crimping seemed to do about as well when headspacing on the bullet. But again, this was for lead bullets. It made no difference to jacketed bullets that I could detect.

But I never ran the hard lead bullet roll crimps some of the old bullseye shooters had liked. I keep meaning to experiment with it, but have a hard time bringing myself to shorten the case life as that inevitably will. But curiosity will undoubtedly win out in the end.

[zeke]

One situation where am concentrating on firm crimp is revolver rounds. Not just the ones loaded with slow burners, but the larger capacity cases (45 Lc, 44 mag, 44 special) loaded with lighter cast/jacketed bullets at low to mid range velocity's.

For the powders being used, Hp-38, Red Dot and Unique, the accuracy was improved. Am GUESSING it is related to keeping bullet seated till pressure rises a little more, or improving the efficiency of the powder burning. And by firm am referring to using LFC or Redding profile crimp to the point there is a slight flattening of the leading edge of the roll crimp (depending on cannelure).

While still crimping pistol and rifle rounds that use a taper crimp, it is slight and maybe more force of habit. Perhaps it is unnecessary, perhaps it helps insuring very slightly more reliable feeding? Still firmly crimping the lever action rounds, that call for it.

[cryogenic419]

From a post on Facebook from the Army Marksmanship Unit they do weekly covering reloading topics. This post is from 12/17/14

PISTOL AMMO: yes, we do develop our own match pistol ammo. We tailor it to the various bbl-manufactures and chamber types used in each new cohort of match pistols. While there are many combinations of powder, primer, bullet and case that can potentially give winning accuracy, we do find one aspect that is often overlooked. Taper-crimp of 9mm and .45 ACP match ammo, in particular, can be an important contributor to accuracy. Once we have promising combinations identified, we will test samples (10-20 rds each) with crimp varying in 0.001" increments. Often, this will be a deciding factor on which combination we will adopt, and results can be dramatic.

[Bart B.]

Note the US Army AMU's best accuracy with their best pistols at 50 yards is in the 3 MOA range as far as I've been able to find.

Taper crimps on pistol ammo's mostly used to get consistant powder burns and muzzle velocities, according to what a Remington rep told me long ago when their .45 ACP match ball and wadcutter ammo was in such demand. Taper crimps minimally distort bullets anyway.

Pistol ammo typically has a lot more bullet runout angle than rifle ammo has. So it's understandable that seating bullets to the lands would improve accuracy. Rimfire match ammo and chambers are set to do that.

[44 AMP]

Crimping revolver rounds isn't just for igniting buckets full of slow powder, its to prevent bullet pull, as well. Bullet pull in a revolver, or "jumping the crimp" is a serious matter in some guns with some loads, not so much in others.

I don't recall ever seeing a lead .38 round that "jumped crimp", even when not crimped (I will admit to not firing many from a snub nose), out of a regular size revolver. Bullets jumping forward in the cylinder seems to show up only when you get to heavier recoil levels and/or bigger caliber, heavier bullets.

I do crimp .38Spl loads, I probably don't need to, but I do.

I once loaded a cylinder of uncrimped .45 Colt for a test. 250gr swc, Unique. 7.5" Ruger Black hawk. Standard level load. After firing 5, #6 was examined, and the bullet had noticeably moved forward. SO, I always crimp .45 Colt. And .44 mag and .357, and ....

[buck460XVR]

I've never found a crimp helpful with jacketed pistol bullets.

While this is my experience with auto rounds(altho the removal of the flare, generally referred to as "crimp", is important for proper feeding), I have found that with magnum revolver rounds using slow burning powders like H110/W296, a consistent firm crimp is needed for consistent burning and best accuracy. At the lower end of their load recipes, some claim the lack of a firm crimp could result in a squib load. With bullets being used for revolvers generally having a cannelure or "crimp groove", I've always assumed crimping them had some significance or there would not be a cannelure.

[oldpapps]

"How much difference does crimp make in the performance of the bullet?
I'd assume that more crimp means more pressure build-up before the bullet starts to move down the barrel.

Does this increase in pressure mean an increase in speed?

More pressure means more case expansion . . . right? Could that cause more failures to eject in a semi-auto."

I've been trying to not get involved with this one. I gave in.

Mr. nick and Mr. B. as well as several others have covered your question well.

It is all a matter of need and timing.

Your first question - performance.
Yes, using or not using a crimp can make some differences.
Your second question - increase in pressure mean an increase in speed.
Yes, higher pressures relate to higher velocities.
Your third question - More pressure means more case expansion.
Yes, that is the way things work.
Follow up to the last question - cause more failures to eject.
That makes scene.

Now to the reality of it all.
A crimp can increase the bullet pull out of the case. I have found that too much role crimp will actually reduce the bullet pull.
Why would we want to increase the bullet pull? To hold the bullet in place during loading and firing. The bullet nose hits the feed ramp and gets pushed back into the case - not good. Or in a tubular magazine, the spring pressure just shoves the bullets deeper in the case, not good. During recoil, rounds not being fired react to inertia and the heavy bullets move forward in the casting. This could lock up a cylinder, not good.
Additionally, some powders like more pressures to get going better. A solid bullet pull can make for a better and more consistent powder burn with some powders.

So we have the pluses of less bullet set-back, reduced bullet walking and better powder burn.

We also have the not so good effects. The bullet must have a canalure to properly be crimped. Alterations to the bullet, cutting/pressing the canalure or swagging the bullet to role the brass in can cause disruption of the quality of the bullet and load.
The brass must be the same length or shoulder set back on bottle necked cases may occur.

Remember the resistance of the crimp is over came very quickly and at hardly any pressures compared to the final pressures of any round being fired. So, all of those 'yeses' above are correct but not necessarily related to a crimp.

So what to do?
I answer is no crimp unless needed. That wasn't much help was it.
I shoot lots of round out of AR type weapons and don't crimp any. (Exception - 300 BlackOut with powder coated lead gets a FCD-taper crimp.)
I shoot lots of .45 ACPs and wimp load .44 Mags and don't crimp any.
I do crimp, FCD, .40 S&W loads. Why? The die came with the die set.

But!
I put a medium crimp on my heavy .44 Mag loads. Why them? I have never had any bullet walking, so that's not it. I have found that substantial charges of slower burning mag pistol powders do better with a crimp. I know my copy and paste from a spread sheet doesn't do well. Your getting one any way.

44 LSWC 240 2400 O 17.5 1256.2 S&W M29-2 9/25/13 10 rnd tested no crimp
44 LSWC 240 2400 O 17.5 1291.1 S&W M29-2 9/25/13 10 rnd tested medium crimp

(That was the last of my 'O' - Old - Hercules brand 2400.)

Standard warning: Safe in my weapon/s the way I load them. May not be safe in your weapon/s. All ways defer to known and trusted loading sources.

The 10 test samples give a tendency of 35 FPS greater velocity and as best as I can tell (macular degeneration) a little better accuracy.

For those rounds that don't need a crimp, it is just another variable that is not needed.

I hope I have helped. If not, well continue on.

Load with care and enjoy.

OSOK