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Thread: Tumbling

  1. #1
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    Tumbling

    Just fired my first few 6mm ppc at a target 50 yds. The holes in the paper indicate bullet tumbling. Where have I gone wrong?
    Thanks

  2. #2
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    So, generally, the three variables are:

    1. What is the twist of the barrel?
    2. how long is the bullet?
    3. How fast are you shooting it?

    As for #1, don't take the manufacturer's word. Measure it with a cleaning rod and a tight patch. For #2, if you don't have calipers, what is the brand & weight of the bullet? #3 is hard to estimate without a chronograph, but given an *average* barrel and load data, we could make a guess.

    This for dynamic stability issues. And just BTW, usually before bullets tumble, they group into about 8 inches at 200 yards. You getting big groups?

    Another possibility (but unlikely in a 6PPC) is the bullets are coming apart, and what you're seeing is the print of the jacket. Again, they won't group. As the usual culprit with bullets coming apart is heat, this is unusual in a PPC -- it's hard to get enough powder in the case, and the bullets have a shorter bearing surface, to build up that much heat.

    As always, there is a faint possibility it's neither of these, and you have a head-scratcher that will require a gunsmith going over the rifle.

    Good luck with it
    Last edited by Charles E; 08-08-2013 at 09:56 AM. Reason: typos

  3. #3
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    Quote Originally Posted by Badmouser View Post
    Just fired my first few 6mm ppc at a target 50 yds. The holes in the paper indicate bullet tumbling. Where have I gone wrong? Thanks
    Here's a quick and dirty from right off the internet:

    Bullets fired from a rifled firearm should NOT tumble in flight. If they do so, it is a sign of problems. This may be caused by different matters that all relate to the rifling in the firearm, and the bullet.

    First is rifling is badly worn, if the bore is oversized for the bullet, or if rifling is badly fouled with lead/ jacketing metal/powder deposits, the bullet will not be gripped by the rifling and spun, and may not be stable in flight (tumbling) Try cleaning the bore properly, and check for excessive wear at the muzzle.

    The second condition is the wrong weight/length of bullet for the rate of twist of the rifling. A fast rate of twist, combined with a very fast bullet, MAY result in the bullet "stripping", and not being spun by the rifling. Or a very heavy, slow bullet may not be spun fast enough to stabilize the bullet.

  4. #4
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    What bullet?
    What powder at what charge?

  5. #5
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    I'm not sure that the cause of bullets coming apart is heat. Or to put it another way, IF it's heat then there's enough heat generated in even a short pistol bore to melt the lead.... witness this video http://uverseonline.att.net/tv/show/...E_253613_ASSET


    IME bullets pop from over-rotation

    al

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    Quote Originally Posted by alinwa View Post
    I'm not sure that the cause of bullets coming apart is heat. Or to put it another way, IF it's heat then there's enough heat generated in even a short pistol bore to melt the lead.... witness this video http://uverseonline.att.net/tv/show/...E_253613_ASSET


    IME bullets pop from over-rotation

    al
    Al, it is almost always a combination of factors, with the biggest culprits being heat and rate of twist. Tooley has done a fair bit of work on why bullets come apart, as has R.G. Robinett. Both got involved because of some long-range flat base bullets -- a 121-grain 6mm for Randy, a 140-grain 6.5 for Dave.

    In the case of the 140-grain 6.5's, we're pretty sure it was a problem with the jackets, though all that means is these long-shanked bullets (more heat from friction) in fast-twist barrels were stressed beyond what those particular jackets could take.

    And not always, of course, Using a 9-twist 6.5/06 AI barrel, I got smallest group of the year at Hawks Ridge (1,000 yards) with those bullets in 2005. Beyond that one group, Dave, Steve Shelp and I got any number of match wins. And too many losses, where there were only 4 on. Or one on.. Dave set up a sheet of paper 15 feet in front of the muzzle, and you'd see where, periodically, molten lead hit, as well as the jacket -- or occasionally a couple pieces of the jacket. Switching to plywood, Dave even figured out where the bullets were coming apart -- just above the point where the shank merges into the ogive. And of course, that fits with Henry's calculations.

    It was similar with the 121-grain 6mms, but Randy took a different path. He got them to stabilize in a 10-twist barrel, (Dasher) using that coating they now favor up there (something other than moly, but same general idea). Believe they were even able to get them to work in with a bigger case than the Dasher, same approach.

    OK. I'm assuming with his 6PPC the OP isn't using a long-jacketed bullet, and it probably isn't, what, a 5-twist barrel? That's about what it would take with PPC case.

    It *could* be defective jackets, but even here, they're usually good enough that problems don't show unless you're stressing them with both unusual heat & twist.

    Can't really even begin to diagnose the OP's problem without more information. And best would be to be there, at the range, with a borescope...
    Last edited by Charles E; 08-10-2013 at 12:10 AM. Reason: wrong date

  7. #7
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    Quote Originally Posted by Badmouser View Post
    Just fired my first few 6mm ppc at a target 50 yds. The holes in the paper indicate bullet tumbling. Where have I gone wrong?
    Thanks
    Verify barrel twist rate. Try different bullets.



    Glenn

  8. #8
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    All good advice

    I will start with rate of twist - and a clean barrel.
    Thanks All

  9. #9
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    Could you post a picture of the holes that make you believe the bullets are tumbling?

  10. #10
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    Quote Originally Posted by Charles E View Post
    Al, it is almost always a combination of factors, with the biggest culprits being heat and rate of twist. Tooley has done a fair bit of work on why bullets come apart, as has R.G. Robinett. Both got involved because of some long-range flat base bullets -- a 121-grain 6mm for Randy, a 140-grain 6.5 for Dave.

    In the case of the 140-grain 6.5's, we're pretty sure it was a problem with the jackets, though all that means is these long-shanked bullets (more heat from friction) in fast-twist barrels were stressed beyond what those particular jackets could take.

    And not always, of course, Using a 9-twist 6.5/06 AI barrel, I got smallest group of the year at Hawks Ridge (1,000 yards) with those bullets in 2005. Beyond that one group, Dave, Steve Shelp and I got any number of match wins. And too many losses, where there were only 4 on. Dave set up a sheet of paper 15 feet in front of the muzzle, and you'd see where, periodically, molten lead hit, as well as the jacket -- or occasionally a couple pieces of the jacket. Switching to plywood, Dave even figured out where the bullets were coming apart -- just above the point where the shank merges into the ogive. And of course, that fits with Henry's calculations.

    It was similar with the 121-grain 6mms, but Randy took a different path. He got them to stabilize in a 10-twist barrel, (Dasher) using that coating they now favor up there (something other than moly, but same general idea). Believe they were even able to get them to work in with a bigger case than the Dasher, same approach.

    OK. I'm assuming with his 6PPC the OP isn't using a long-jacketed bullet, and it probably isn't, what, a 5-twist barrel? That's about what it would take with PPC case.

    It *could* be defective jackets, but even here, they're usually good enough that problems don't show unless you're stressing them with both unusual heat & twist.

    Can't really even begin to diagnose the OP's problem without more information. And best would be to be there, at the range, with a borescope...
    Holey Cow..... Good Post!



    al

  11. #11
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    Picture of hole

    Do not have a picture, but the hole was about 1/2 to3/4" long an angled up to the left. This was at 50 yards

  12. #12
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    Badmouser,

    I had the same problem in my then-new Shilen DGV 8-twist, 4-groove 6BR. I started out with 87 grain V-maxes and THEY WOULD NOT SHOOT! I had key-holing and absolutely no accuracy. My best "group" at 100 yards was over an inch and a half! I finally got wise and switched to Berger 80 grain flat base match varmints and I now get better than 1/4 MOA consistently, and better if I really do my part. With the Bergers, my 6BR is pretty happy with loading into or out of the lands, and 27 +/- 1.2 grains of 8208-XBR.

    So sum it up - try some different bullets to start and good luck!

    Dennis

  13. #13
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    Quote Originally Posted by DeltaBravo View Post
    Badmouser,

    I had the same problem in my then-new Shilen DGV 8-twist, 4-groove 6BR. I started out with 87 grain V-maxes and THEY WOULD NOT SHOOT! I had key-holing and absolutely no accuracy. My best "group" at 100 yards was over an inch and a half! I finally got wise and switched to Berger 80 grain flat base match varmints and I now get better than 1/4 MOA consistently, and better if I really do my part. With the Bergers, my 6BR is pretty happy with loading into or out of the lands, and 27 +/- 1.2 grains of 8208-XBR.

    So sum it up - try some different bullets to start and good luck!

    Dennis
    I'll guess this is a fat neck chamber

    VLD's need minimal neck clearance

  14. #14
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    al,

    I'm not sure what a "fat neck" chamber is. Mine is .270 for the Lapua blue box brass and a loaded round is right at .268, giving about 1 thou all around. Is that too "fat" and if so, why do the flat base Bergers shoot so well (for me anyway)?

    Dennis

  15. #15
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    I had the same problem in my then-new Shilen DGV 8-twist, 4-groove 6BR. I started out with 87 grain V-maxes and THEY WOULD NOT SHOOT! I had key-holing and absolutely no accuracy
    This is odd. A fair number of us shoot 6mm 105-grain bullets, some of which are VLDs, out of an 8-twist barrel. At least one guy, Jason Baney (Pennsylvania 1-KB) with a plain 6BR.

    My best "group" at 100 yards was over an inch and a half!
    When I say "poor grouping" I'm talking about 8-inches at 200 yards. Pie-plate size groups. 1.5 nches don't qualify ;-)

    You're right that if a bullet won't group, move on. And there is no reason to shoot the 100+ grain bullets if you're not going to shoot 600+ yards. But the bulk of them should not only work, but work well. See R.G. Robinett's page for 6mms & twist:

    http://bibullets.com/products-2/

    To quote on the heavier 6mms:

    6mm- 95 Gr. FB, BC-.45 : Made in Niemi carbide dies; 11 ogive/.052″ meplat; .2435 pressure-ring; .2433″ shank. Ideal twist: 1:10″ twist; works well in twist rates as fast as 1:8″.


    6mm-104 Gr. BT, BC .54 : Made in Niemi carbide dies; 11 ogive/.052″ meplat; .2433″ shank; 10 Deg. x .175″ BT. Ideal twist rate: 1:8.3″; twists faster than 1:8″ not recommended.
    I bring Randy up becase he's always made great bullets, but some of his just set the new all-time small group record of .0077, so he's esp. hot just now.

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