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Thread: Does Crimping effect bullet runout?

  1. #1

    Does Crimping effect bullet runout?

    The short answer is No. It doesn't appear to.
    I spent Christmas day loading 100 rounds of 300 AAC Blackout with Speer TNT 125 grain bullets. Using a Hornady Bullet Run out Gauge I ran the run out on each cartridge before and after crimping. The average runout was +/- .001 to .0015 inches. This didn't change with moderately heavy crimping -Which is to say it neither got worse nor improved with crimping. I used shortened Federal 5.56 brass that was FL resized, trimmed, had the case mouth flared to accept the bullet base, and then seated. The Dies were Lee and the press was my Forster. A Lee FCD was used to crimp.

    As a further aside on the necessity of crimping properly, I read an astoundingly unscientific article on reloading for the 300 AAC Blackout by the NRA here in the US. The author reported poor accuracy using a taper crimp from the Redding dies he was pimping for the piece; writing off poor accuracy with bullets such as the Nosler 125 gain Ballistic Tip to the lack of a crimping groove. I have loaded these same bullets with identical loads as his but used a Lee FCD and got half MOA. Save the taper crimp for light loaded 45 ACP or 44 Special gallery loads...~Muir
    Last edited by Muir; 26-12-2015 at 00:40.

  2. #2
    I use the Lee FCD on all my reloading, I load 300aac using a subsonic load of 3.4gr of Viht N310 with a 168gr cast boolit and it produces almost single hole accuracy (at 25m). Same with the 44magnum, my "indoor range" subsonic load of 4.8gr of TiteGroup or 6.5gr of NobelSport GM3 and a cast 200gr gives a very accurate load. I haven't any way of measuring runout but I'm very happy with the results from the FCD
    There's room for all Gods creatures ........... right next to the mash and gravy

  3. #3
    My own findings are no, runout is not improved, nor particularly changed by use of the Lee Factory Crimp Die. Other techniques are used to maintain low case sizing & bullet seating runout. .... But, it does even out neck tension and that in simple terms aids pressure consistency. So all other brass and brass prep being carried out diligently, the nett result is usually improved consistent accuracy...and that is what I find, so I'm happy to continue using the Lee FCD.
    Last edited by deeangeo; 26-12-2015 at 08:24.
    Blaser K95 Luxus Kipplaufbüchse .25-06Rem. Zeiss 8x56, 110gn Nosler Accubond = Game Over!

  4. #4
    I should have premised the post by saying that I have heard both; that crimping somehow 'straightens' the bullet and improves run out and the opposite, that it ruins concentricity. Whether it is a benefit or not is a whole 'nuther subject. While the use of crimping is gaining acceptance, it is still frowned upon by a lot of people as something that destroys accuracy and ruins the cartridge case.~Muir

  5. #5
    Quote Originally Posted by Muir View Post
    I should have premised the post by saying that I have heard both; that crimping somehow 'straightens' the bullet and improves run out and the opposite, that it ruins concentricity. Whether it is a benefit or not is a whole 'nuther subject. While the use of crimping is gaining acceptance, it is still frowned upon by a lot of people as something that destroys accuracy and ruins the cartridge case.~Muir

    I have done a lot of testing with crimped and uncrimped rounds and have found the crimping to half my es and sd. I use a device to reduce runout to under a thou. Then I will crimp the round afterwards. The runout does not change at all- I check every round. I crimp all my loaded rounds as I find my groups are smaller and the consistency gets better.

  6. #6
    Quote Originally Posted by murph View Post
    I have done a lot of testing with crimped and uncrimped rounds and have found the crimping to half my es and sd. I use a device to reduce runout to under a thou. Then I will crimp the round afterwards. The runout does not change at all- I check every round. I crimp all my loaded rounds as I find my groups are smaller and the consistency gets better.
    Holy Smokes! You check every round you load?? That's devotion! I just checked these because it was a -8F cold, snowy Christmas day and I had five pounds free time and a half an ounce of curiosity. That concentricity gage hangs on the wall of my reloading room and I just happened to look up and see it there!

    I agree with you about the reduced ES and SD 100%. I have gone to great lengths to prove it, with a number of weapons, though ammo makers have always known this; which is why they put some form of crimp on their ammunition. I also crimp everything. I will have crimp dies custom made if Lee doesn't already offer it. Two of the greatest myths hovering around the bullet seating aspect of reloading: You must seat to X-thou from the lands, and crimping has no value. ~Muir

  7. #7
    Quote Originally Posted by Muir View Post
    Holy Smokes! You check every round you load?? That's devotion! I just checked these because it was a -8F cold, snowy Christmas day and I had five pounds free time and a half an ounce of curiosity. That concentricity gage hangs on the wall of my reloading room and I just happened to look up and see it there!

    I agree with you about the reduced ES and SD 100%. I have gone to great lengths to prove it, with a number of weapons, though ammo makers have always known this; which is why they put some form of crimp on their ammunition. I also crimp everything. I will have crimp dies custom made if Lee doesn't already offer it. Two of the greatest myths hovering around the bullet seating aspect of reloading: You must seat to X-thou from the lands, and crimping has no value. ~Muir

    My home made concentricity guage / straightener is very quick. About 5-10 seconds does each round. I only handload 223 so it isn't all that time consuming. This machine is set up for 223 only. There's maybe only one in ten rounds that actually need straightened. The rest I just go through and any over a thou of runoff I give them a go. I also use fairly light neck tension (1 thou) which leaves stress on the neck to the minimum. Here's a picture of the device:

  8. #8
    Muir and I had a discussion about this last year. I am a big believer in the crimping, and the Lee Factory Crimp die.

    Most people would think that with the huge pressures, that the resistance of the crimp would be miniscule, but it is not, and here is why:

    In the first few microseconds of powder burning, the pressure spikes, then decreases as the bore volume increases with the acceleration of the bullet, because pressure decreases directly with the volume increase ( linearly ). And pressure is the force on the bullet which is accelerating it down the bore.

    If one bullet is loose, it will be pushed out by the primer ignition, into the lands, changing the case volume and moving the pressure spike in terms of time and position in bore travel. Then the tight one will be closer back. Result = more variation in MV shot to shot, larger ES and larger groups.

    Accuracy and precision of the rifle and loads means consistency, repeatability. In ammunition, that means consistent and uniform brass of the same volume, the same primers, the same lot of powder charged to the same weight, bullets seated to the same depth in case necks of the same tension. The easiest means of getting close to identical neck tension is a proper Lee Factory Crimp.
    Last edited by Southern; 28-12-2015 at 03:50. Reason: additions

  9. #9
    Southern is100% correct. While this is going on, and as a part of this, the burning rate of the charge in that particular case is being established, which also influences pressures.~Muir

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