.223 Barell twist rates

B17-P51

April 28, 2006 17:39

Is the 1 in 9" twist rate in an AR type rifle better for 55-60 gr pills and the 1 in 8" better for the 69 gr.?

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  sandwarrior

  April 28, 2006 21:10

  B17-P51,
  
  The 1-9" twist will stabilize the 69 gr. bullets if you load them up the potential velocity they belong in, 2900 fps or better. Which the .223 is perfectly capable of. The 1-8" twist will stabilize them at lower velocities. I will go on to say the 1-8" twist is more for the 77-80 gr. range of bullets.
  All that being said remember it's the length of the bullet not the weight that is really the key to finding what will stabilize and what won't. Some basic formulas for finding twist rates are the Greenhill which is: 150xD(sq)/L for under 2800 fps and 180 x D(sq)/L for over 2800 fps. A variation of that called the velocity formula, which is: .06 x V(fps)x D(sq)/L.
  
  Once you buy a rifle and want to change bullets try working the velocity formula like this V=TL/D(sq)/.06. This will help determine the best velocity needed for each bullet, since you already have two givens: bullet length and barrel twist.
  
  This will give you a general range to work in for twist, bullet length and velocity needed. I say general because you won't get exactly the best performance from these formulas. They are too general for each bullet type. nononsense pointed me in the right direction not too long ago with a site that gave all the variables to measure when figuring twist and calculating that path of the bullet over given distances. Unfortunately, I lost it when my nieces and nephews came up to visit and we were shortly thereafter left with a nasty virus that took some creative downloading to get rid of. Obviously, files went away and that was one of them.
  
  hope that helps

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  Gunslinger2005

  May 05, 2006 16:51

  The guys over at AR15.com have a pretty good site called the Ammo Oracle about ammo and related topics. Here's a link to the topic about twist rates and various types of ammo: http://www.ammo-oracle.com/body.htm#twists

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  dljacko

  June 16, 2006 15:04

  Basically the 223 was designed to fire a 62gr military projectile and I think it had a 1in10 twist. All of the other 22 calibres up until then were only firing light weight projectiles and had 1in12 or 1in14 twists to stabilise those. Because of the proliferation of heavier projectiles on the market now a lot of European manufacturers are going for 1in8 twists. Seek information from those who shoot target with these heavier projectiles before committing money to a purchase either way. They will probably be able to put you on the right track to matching twist rate to bullet weight and type from their experience on the range.
  Regards.

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