[SquintBeastwood]

Hi,

I have a SR556TD I bought a year or so ago and I am trying to find out once and for all what the barrel twist rate is. I called Ruger and was told by the woman it is 1:8 but I get conflicting info---is it 1:8 as she stated and stated on Ruger.com or is it 1:9 I hear around the 'net? Was there a change by Ruger at some point from 9 to 8? I need to figure out the best bullet wt without spending money on ammo needlessly. Thanks

 

[SquintBeastwood]

I finally got the skinny on twist rates. It was an aha moment too as they did change with the later models. The 5901 TD has a 1:9 twist rate (mine) and the 5924 has the 1:8. So there ya go for anyone that had the same question which apparently is no one. Oh well.

 

[VThillman]

Hah. Yeah, oh well. Just means it can stabilize a slightly heavier (longer) bullet, or maybe blow up a light one at a little slower velocity, I guess. Seems like it was not long ago I was reading praise for 1:8.8. What happened to that?

 

[SquintBeastwood]

Seems that many AR's are coming with 1:7. Why such a fast twist? Seems like all one can shoot would be the heavy bullets limiting one to more expensive ammo and 223 to boot. Probably wouldnt stabilize 556 55 gr which is so abundent.

 

[spikedriver]

My understanding is, 1:7 should (note the use of "should", not "will"...) work for 55 grain bullets but 62 to 77 grain is ideal. 1:9 is ideal with 40 to 50 or 55 grain. (My AR is 1:9 and prefers 50 - 55 grain.) So, 1:8 should work well for 55 and 62 grain ammo.

 

[greg_r]

VThillman hit on the truth. Required twist rate is determined by the length of the projectile, not the weight. A 1:9 twist might do just fine with a 62 grain lead core, but may not stabilize a 62 grain solid copper. The solid copper bullet will be longer.

But I have this chart see......? The charts you commonly see out there work, but only because they are based on lead core bullets. Lead core can only be so long for a given weight in a given caliber. Start mixing things up and suddenly those charts don't work so well.

 

[greg_r]

I will add that I do believe there is an optimum twist for a given bullet, but the over stabilization because of a too fast twist is much overhyped. I have had bullets turn to dust out of a 22-250 or 220 Swift when they hit the atmosphere just outside the barrel, but not in a 223. I'm sure it's happened, just not with me. And most people will not see much increase in accuracy in different twists as long as the twist is fast enough to stabilize the bullet to begin with.

 

[SquintBeastwood]

I came across this formula a while ago that figures out the rpm for a given bullet for various twist rates but one needs to know the mfr's optimum rpm for that particular bullet and also have to consider barrel length but since many cartridge mfr's test most ammo thru a 24" barrel and velocity changes with barrel length and one has to know what the loss or gain is to make the formula work and if you dont have a chrono to check it then it is a guessing game.

The formula for RPM: (Muzzle velocity in fps) X 720 ------------------------------ = RPM of bullet (Twist in inches)

Damn, I used to think ya got a gun and ya get ammo and ya go shootin but now it has become a science to buying the right ammo to get the most out of my firearms and now I have become somewhat obsessed with matching the correct bullets for the twist rates for the best precision and accuracy. Ahhhh, maybe I should forget all that crap and just go shoot and live with the fact that some guns just don't like some ammo.

 

[SHOOTER13]

Almost every AR barrel I've seen has the twist rate roll marked right on it...right next to the caliber mark.

My Olympic Arms AR is chambered in 5.56 NATO and has a twist of 1:9





Works just fine with the 55 and 62 grain Lake City ammo I buy...

 

[jigs-n-fixture]

All of my SR556 uppers have the caliber and twist rate stamped on the top of the barrel, just in front of the piston assembly, and are 1:8. The SR556-6.8 SPC, is the only one that doesn't have a twist rate stamped on it.

The twist rate required to stabilize a bullet, is a function of the length of the bullet, the location of the center of gravity of the bullet, and the velocity. Back when all bullets were primarily lead, with a thin copper jacket to minimize bore fouling, the weight could be used. But with the great number of variations in bullet construction and metals used today, you really need to look at the length, and CG.

 

[SquintBeastwood]

All of my SR556 uppers have the caliber and twist rate stamped on the top of the barrel, just in front of the piston assembly, and are 1:8. The SR556-6.8 SPC, is the only one that doesn't have a twist rate stamped on it.

The twist rate required to stabilize a bullet, is a function of the length of the bullet, the location of the center of gravity of the bullet, and the velocity. Back when all bullets were primarily lead, with a thin copper jacket to minimize bore fouling, the weight could be used. But with the great number of variations in bullet construction and metals used today, you really need to look at the length, and CG.

I am starting to learn that length of bullet is more important factor. I'll have to check my upper. Never noticed twist rate last time I had barrel.off.

 

[SquintBeastwood]

Almost every AR barrel I've seen has the twist rate roll marked right on it...right next to the caliber mark.

My Olympic Arms AR is chambered in 5.56 NATO and has a twist of 1:9





Works just fine with the 55 and 62 grain Lake City ammo I buy...

I did not know that, I'll have to check. When I bought the rifle the sales guy told me it would work with 55 -70 gr but after reading more I think you are right on 55-62 gr being the best suited for that twist.

 

[paulruger]

Check it yourself!

 

[SquintBeastwood]

Thanks gentlemen, I have indeed confirmed my rifle's barrel twist as indicated on the barrel. It is 1:9 and I never knew the twist rate was imprinted with the caliber, I thought just the caliber was on them. I wish I had the 1:8 though.

 

[nick7274]

All my 5.56 rifles have a 1:7 twist rate. This is not a coincidence. I only get 1:7 because it is the best rate to have for a broad range of bullets. I shoot 55-77 grain bullets with very little difference in accuracy. In some calibers the twist rate can make a big difference but it is hardly noticeable in .223. The 1:7 is fast enough to stabilize the larger grain bullets and is not too fast for smaller grains.