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School me.

Discussion in 'Rifle Discussion' started by shockercustoms, Jul 21, 2009.

  1. shockercustoms

    shockercustoms Forest Grove, Oregon Member

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    I have searched for awhile to find out the complete understanding and differences in barrel twist and grain recomendations(i.e.)1 in 9 twist vs 1 in 14 twist. So someone please shed some light on this subject for me. I know there are a few others that are curious as well. If there is a post about this already can someone point me in that direction.
    Thanks in advance,
    Mike
     
  2. speelyei

    speelyei Willamette Valley Active Member

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    twist refers to the number of full revolutions the rifling makes in a given distance. 1:12" makes a complete turn in twelve inches, 1:9" makes a complete turn in nine inches, and 1:7" in seven inches.

    a faster twist turns the bullet faster. A faster spin will stabilize a heavier bullet. A .223 rifle will stabilize a bullet up to about 55gr adequately with a 1:12 twist and give good results to a given yardage, depending on a variety of factors.
    Lets say you have a .223 bolt action rifle with a 1:12" twist. You shoot a 55 gr bullet, and you can consistently shoot dime sized 5 shot groups fom a rest at a hundred yards. At 200 yards, they all fit under a nickel, and at 300 yards, under a quarter. That would be some pretty good shooting. But at 400 yards, the groups seem to open up. Wind plays havoc, you just can't get the groups to stay tight...
    You adjust your rest, your breathing, your optics... soon you realize, the problem isn't you, it's some problem with the load or bullet. Chances are yo would determine that a combination of factors were at play... the bullet speed has slowed down, the wind is having more of an effect on the slowed projectile, there simply isn't enough inertia to carry the little projectile accurately to the target at those distances.
    The obvious move is to load a hotter load with a heavier bullet. Now, I am no pro or guru, but I do know a few things.... a hot load will only get you so far. Pushing loads can have disastrous consequences, and often you find that exceeding the maximum load only gains you a few FPS (feet per second). Since the diameter of all .223 bullets is .224, heavier bullets are necessarily longer. The longer, heavier bullets have a higher ballistic coefficient. In other words, they buck the wind better and carry more speed and spin at longer distances.
    So you go buy some 80gr bullets, and load them with a middle of the road load, and you go and shoot them at 100 yards. The groups are terrible, some of the bullets land sideways and leave a distinct "keyhole" hole in the paper. The problem is that the heavy bullets are not being twisted fast enough to stabilize them, and they are flying comparativley erratically to the lighter bullets which were stabilized fine by the slower twist rate of the rifle.
     
  3. speelyei

    speelyei Willamette Valley Active Member

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    A real scenario:
    F-class competition in .223. 1000yds prone from a rest or bipod. A common rifle would be a bull-barelled .223 with a full floated barrel and a bedding job in the stock. The bases might be designed with 10 MOA of slope to allow enough adjustment for the long distance. The shooter would be a hand-loader who had painstakingly trimmed his brass, mic'ed and weighed everything, weighed all the powder loads on an avordupois scale, and seated the bullets to contact the lands of the rifling in a very specific manner. He would have worked up a specific load with a chronograph and tried multiple powders and bullet weights. After shooting dozens of test targets, he might find that his rifle shot best with a certain combination above all others, for instance,
    24" Bull barell, 1:8 twist, 75gr A-Max bullets, IMR 4064, OAL 2.260"
     
  4. Andy

    Andy Aurora, OR Active Member

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    Shockercustoms,

    Are thinking of buying something with a specific twist rate, or do you already have it?
     
  5. shockercustoms

    shockercustoms Forest Grove, Oregon Member

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    Thank you very much. I didn't expect anyone to be bothered to type that much, but I think I have a pretty good understanding of what your saying.
    :thumbup:

    I'm not to the buying part yet, I was just looking for a summary of how things work. I like seeing whats out there and what my options are before I learn the hard why by buying something that won't be up to my expectations. Twist and grain loads were not openly discussed in the military.
     
  6. westernsky20

    westernsky20 Portland, OR Member

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    Are there any good books or web sights on this and simialar subjects anyone can recommend?
     
  7. speelyei

    speelyei Willamette Valley Active Member

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    yes.

    http://www.chuckhawks.com/index2.guns.htm
    Chuck Hawks gives a lot of no-nonsense information , and down-to-earth reviews of equipment.

    http://www.snipercentral.com/forums/
    Supposedly for the Tactical LEO/Military shooter, most of these guys just enjoy shooting very accurate bolt action rifles in a variety of calibers. "Search" is your friend on this forum, they'll jump all over you for asking the same question the last 1,362 people did. Lots of good information, some of these guys are the real deal.

    http://www.frfrogspad.com/#Shooting
    Tons of info. I just printed off most of this information and put it in the bathroom for a little light reading.

    There are a lot of books out there. Amazon.com will save you a lot of $$
     
  8. Jamie6.5

    Jamie6.5 Western OR Well-Known Member

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    General rule:
    The longer the bullet you want to shoot, the faster your twist rate needs to be. We often say heavier bullets, but the length is actually the determining factor, because bullet design/ogive radius changes length/weight ratios.
    A 140gr SMK in 6.5 will stabilize with a 1:9". When Sierra added 2 grains they made it MUCH longer. The 142gr SMK needs a 1:8". A 140 Berger likes the 1:8" too.
    I have a URL somewhere that explains the dynamics of it all. If I find it I'll post it.
    Hope that helps.
     
  9. Jamie6.5

    Jamie6.5 Western OR Well-Known Member

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  10. shockercustoms

    shockercustoms Forest Grove, Oregon Member

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    Thanks alot for the info guys.:thumbup:
     
  11. Grizzly_A

    Grizzly_A Portland Metro Area Member

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    Wow, given the topic, and the can of worms that it usually opens, I'm impressed with the responses. Kudos!

    One thing to consider is that every cartridge has an optimal twist rate range, so it's harder to compare the 30-06 with the .223 based strictly on twist rates.

    Usually when you start getting picky about twist rates is when you start needing to do specific things with your gun. (competition, long range target or varmint hunting) This affects what ammo you'll be shooting which depends on your barrel, etc.

    Take the .223. Twist rates for .223 rem are 1:14 - 1:6. You'll find twist rates vary by manufacturer and model within that range. For example Remington has a 1:10 for some rifles and 1:9 for others.

    Here's another consideration to the twist rate argument that I stole off another forum:
    "given a rifle barrel with a 1 in 12 twist causes a bullet traveling 3000 fps to spin at the incredible speed of 180,000 rpm and a 1 in 9 at 240,000 rpm. If you were to increase that velocity to lets say 3300 fps your 1 in 9 twist is going to be spinning that bullet 10% faster or 264,000 rpm which is in the area that a lead core bullet will come apart due to centrifugal force. My point being that if you were to decide to shoot any bullet that you could drive somewhere in that 3300 fps range with a 1 in 9 twist it will not only not be accurate, and depending if it where a lead core bullet, you could find that the bullet will never reach the target because it vaporized before it ever got there."


    So...don't target shoot with high velocity 45 gr pills out of a 1:7 .223 barrel unless you're not wanting to put holes in targets downrange. :)
     
  12. tac

    tac UK, Oregon and Ontario. Well-Known Member

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    Happened over here in yUK a few years ago with an older Winchester in .220 Swift. Using a well-known maker's 40gr bullets over a top-end load resulted in no holes appearing anywhere on the target at 100m - even after making collimation settings with the scope using my laser bore-sighter.

    We laid down a length of white wallpaper out to about ten feet in front of the muzzle and found that the bullets were converting into coppery powder about a foot from leaving the barrel...

    Easing up on the load slowed things down to bit - still to around 3890fps, since we could actually chrono the things then.

    tac in yUK