Twist rate vs bullet weight

Discussion in 'Big Stuff--7mm, 30 Cal, .338+' started by spudgun, May 23, 2019.

  1. spudgun

    spudgun

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    OK, I have a 308 with a 1:12 twist rate.

    I am shooting Hornandy 168 Match bullets at 2520 FPS.

    Not really getting the accuracy I was hoping for.

    I was aware that most 308 barrels have a 1:10 twist rate but with a 1:12 do I need lighter bullets or higher velocities?
     
  2. Straightshooter1

    Straightshooter1 Gold $$ Contributor

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    Here's a twist rate calculator that can help you with that:

    https://bergerbullets.com/twist-rate-calculator/


    PS: Looks like this bullet is stable for your 1:12 twist based on a bullet length of 1.215 and a BC of .450, so it doesn't look like that this your accuracy problem.
     
    Last edited: May 23, 2019
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  3. spudgun

    spudgun

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    Thanks, looks like the 168 are just in the start of the stability chart. A 1:10 or 180 gr bullet looks better.

    My bullet came in at 1.5 on there SG chart.
     
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  4. D-4297

    D-4297 Silver $$ Contributor

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    You might consider talking to some local F class shooters about improving your load . 2520 is a little on the slower side for a 168gr bullet . Some of the Palma types are pushing 168's up to 2800+ , but that's Palma shooters for ya . :D:D:D:D
     
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  5. Zero333

    Zero333 Silver $$ Contributor

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    How long is your barrel ???

    2,520 fps is slow unless your barrel is around 18"

    Mind sharing your load data for this combo ???

    Like Straightshooter1 mentioned... 12" twist is more than enough for the 168 Horn.bthp.

    I'd go with the Sierra 168 smk if you want better accuracy. Varget / H-4895 are most popular and if you prefer a ball powder I recommend W-748.
     
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  6. chkunz

    chkunz Gold $$ Contributor

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    The M118 and M852 military match ammo velocity is 2550 fps for the 22 inch M14 barrel. The bullet weight for the two bullets is 173 for the M118 and 168 for the M852. The twist rate used by competitors is in the 1 in 10 to 1 in 12 range.
     
  7. spudgun

    spudgun

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    thanks for the info.

    I didn't really want to push the bullet yet. Won this gun, T/C compass, 308 win with 22" barrel at my club raffle.

    I am using H335 ball powder at this time due to it's what I have on hand. I also have other powders that I use for my 6.5 CM.

    My load is 40.0 g H335, CCI primers and 168 Match.

    I did check the lead of the bullet and found that factory 150 FMJ was the worst accuracy, but I had 300 rnd laying around so I thought I would break the barrel in using that stuff and then reload the brass.

    Gauged my chamber and found my jam point was 2.868 with the 168 match. I loaded up some 2.83 and 2.85 OAL 168 gr and my best groups were with the 2.83 OAL. So I'm going to work around the 2.83 OAL to see if I can find a sweet spot.

    Wasn't sure what FPS the 168 bullet likes so that is why I asked.

    I know I can bump up the charge to increase FPS.
     

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  8. Lone Hunter

    Lone Hunter Silver $$ Contributor

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    Are you measuring to the lands CBTO or over all length OAL? Sounds like you are running as a repeater and not single loading either. Your measures are confusing to me.
     
  9. joed49

    joed49

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    I don't think I know anyone that has used H335 in a .308. I've had the best luck with Varget and IMR4064. My .308 is a 1:12 twist but 26". Nosler and Sierra 168 gr MKs have worked well for me.

    Try a different bullet and powder, it might surprise you.
     
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  10. FrankG

    FrankG

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    If it’s a factory barrel it is most likely a 1-12 twist. That is SAAMI spec. There are some factory barrels with a faster twist as well.

    What gun? What barrel is on it?

    That being said a 12 twist will stabilize a 168 with no issues. Sometimes they will even fly in a 13 twist (I’d never shoot one though).

    As been mentioned a different powder/different load.

    How many rounds on the barrel? Have you tried a different bullet? I assume the stock is bedded etc..?

    Have you checked the muzzles crown for any damage? A lot of different things could be going on.

    Need more info.

    Bullet weight does not necessarily dictate twist rate needed. Bullet length is more of a factor. Yes as bullet weight goes up usually it gets longer but not always.

    Later, Frank
    Bartlein Barrels
     
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  11. mike06

    mike06 US ARMY 1970-1977 Silver $$ Contributor

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    Spud I am getting 2780 FPS using 43.0 grains of ArComp BR2 primer 168 Nosler RDF out of my Rem 700 VSSF 308 letting the bullet jump .060 awesome load for that 25 year old rifle 1-12 twist. 3 shots 100 yards.
     

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  12. Kurz

    Kurz

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    We're all sort of jumping into the middle here like we're working with a target rifle instead of a factory hunting rifle. First and foremost:

    Is the barrel free floated?

    Has the rifle been bedded?

    Did you check the crown?

    How is the trigger?

    Is your scope of good quality?

    Are all the various tightening points tightened correctly?

    Is the scope true to the bore?

    Do you really know what the dimensions of your chamber are?

    I would suggest shooting some good quality factory ammo (SAAMI spec) first to see how it behaves.

    Then I would bed the rifle and free float the barrel although it may require some 'up pressure' at the very end of the forearm. Then get the trigger tuned if possible.

    Now go to work creating your tests and loads.

    Respectfully, when a shooting partner and I worked on a competition rifle for a fun shoot we found that the 175 grain bullets were the best without getting up into the real heavy weights. We used the Berger VLD or the Hunter if you prefer, over a load of IMR8208XBR near the top node. Extremely accurate while running somewhere around the 2,650 fps.

    Enjoy the process!

    By the way, it's the length of the bullet which is the component regarding twist.;)
     
  13. Baddog0302

    Baddog0302

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    YES what he said
    IT IS BULLET LENGTH that determines THE TWIST NEEDED !!


     
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  14. spudgun

    spudgun

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    Well, today was a better day.

    Loaded up some Varget loads and went to the range. Velocity was up from 2530 (h335 powder) to 2620 (Varget) FPS.
    2 different OAL lengths 2.83 and 2.84" same bullet (Hornandy 168 HPBT Match)

    Much better groups at 100 yds.

    One thing I noticed was when the barrel was hot, 140 deg or more the better the groups was.

    Load data: 43.0 gr Varget, 168 HPBT Match Horndanady, CCI primers.

    Did not really notice a differance in the OAL one was set at .04" and the other .03" lead jump.

    But the barrel temp was something that I did notice on the groups.

    Yes the barrel was free floating at cold and hot temps.
     

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  15. cmillard

    cmillard

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    When it comes to military rifles and spec ammo, the listed velocity is not at the muzzle, but at 65 feet, so muzzle velocity would be slightly higher
     
  16. theallcineyes

    theallcineyes

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    In general, heavier bullets require faster twist. The smaller the ratio number, the faster the twist. For example, 1:8 is faster than 1:12. The terminology means that the rifling makes one complete revolution in X inches. The rifling in a 1:8 makes one complete revolution in 8 inches.
     
  17. 338 dude

    338 dude Silver $$ Contributor

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    F445D50B-0B06-4717-931D-D34D547B0B3E.jpeg 6B616353-E08E-4F2C-93FB-E93FEEAB8FD0.jpeg
    This is a bullet selection chart from Burger highlighting 30 caliber bullets zoom in and look for bullets where a 1 in 12 twist rate is optimal It looks like you will be limited to a 168 grain bullet or less except for the 185 VLD Hunting or the 185 classic hunter
     
  18. Kurz

    Kurz

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    Since you are apparently getting better results from your newest combination, I suggest that a better approach now is to shoot a ladder test to find the node you need for smaller groups then shoot the next ladder with your length or jump being the variable. Note that a ladder test is better shot at longer distance than 100 yards, I use 300 yards which makes the nodes easier to see.
     
  19. Stepsjframe

    Stepsjframe

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    The 1 in 9 twist is good for up to 70gr bullets. That is the upper range though, and is optimized for the 55-69 grain range. Most varmint 223's that are shooting the really light bullets (40 gr) have a 1 in 12 twist.
     

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