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Posted: 1/10/2006 5:45:07 PM EDT
Well i'm starting to look for an AR upper complete assemply to start my slow build. And i'm somewhat confused about the twist rate. someone explain this too me please. is it better to have a faster twist rate like 1 in 7inches or slowers lik 1 in 9 inches. Give all the feedback you can give me!!
Link Posted: 1/10/2006 5:58:42 PM EDT

Originally Posted By ford_shooter:
Well i'm starting to look for an AR upper complete assemply to start my slow build. And i'm somewhat confused about the twist rate. someone explain this too me please. is it better to have a faster twist rate like 1 in 7inches or slowers lik 1 in 9 inches. Give all the feedback you can give me!!



From the ammo oracle...


Q. What twist rate do I want for my rifle?

Probably 1:9, but it depends on what kind of bullets you intend to shoot.

Special purpose rifles often have uncommon twist rates. For example, if you are building a varmint rifle and want to shoot the short 35 grain, 40 grain, and 50 grain bullets, a 1:12, or even 1:14 twist would be best. On the other hand, long range High Power shooters often select 1:8, 1:7.7, 1:7, or 1:6.5-twist barrels to stabilize the long 77, 80 and even 90 grain bullets used for 1,000 yard competition. Additionally, new testing of heavier rounds (68-77 grains) seems to show that they perform very well in simulated tissue and may be a better defensive choice than 55 grain or 62 grain rounds. The majority of shooters, though, typically shoot bullets of 50 to 69 grains in weight (note that the 62gr SS-109/M855 bullet is as long as a 71 grain lead core bullet) and should select 1:9 twist barrels. At typical .223 velocities, a 1:9 twist will stabilize bullet lengths equivalent to lead-core bullets of 40 to 73 grains in weight.

1:12 twist rifles cannot stabilize SS-109/M855 bullets and 1:7 twist rifles are slightly less accurate with lighter bullets and will often blow apart the thin jackets of lightweight varmint bullets. The 1:7 twist is used by the military to stabilize the super-long L-110/M856 tracer bullet out to 800 yards, but unless your plans include shooting a significant amount of M856, the 1:9 twist rate is better suited for general use.

There is, of course, an exception: if you want to use loads utilizing the heavier, 75-77 grain match bullets currently used by Spec-Ops troops and other selected shooters, you'll want a 1:7 twist barrel. Although military loadings using these bullets are expensive and hard to get, some persistent folks have managed to obtain a supply, and will need the proper barrel twist to use them. Anyone who foresees a need to shoot this ammo should consider a 1:7 twist barrel.


Link Posted: 1/10/2006 6:13:45 PM EDT
[Last Edit: 1/13/2006 6:32:30 AM EDT by theshootersden]
The rate of twist determines the optimum bullet weight/length for a given caliber and speed of the bullet by applying the proper spin on the bullet to prevent the bullet form yawing and pitching... Expressed in terms of the number of revolutions per inch of barrel length, this ratio is commonly expressed by designations such as 1:9, 1/9 or 1 in 9 twist, the 1 represents 1 twist, the 9 represents inches of barrel length... A good rule of thumb is that the heavier, or "longer" a bullet is, the faster the twist rate needs to be to stabilize it in flight, therefore a lighter "shorter" bullet needs a slower twist rate to give proper bullet spin for correct flight... Over stabilization of bullets can occur when larger heavier bullets in a given caliber are fired at very high velocities...

The twist needed depends on what type of shooting you will be doing...

If you shoot long range and want excellent accuracy go with a fast twist and use long/heavy bullets...

If you shoot short range and want excellent accuracy go with a slow twist and use short/light bullets...

If your just planning on plinking or CQB and want acceptable accuracy go with a mid speed twist so you can use a wide range of different length/weight bullets...

In general for the .224:
1/7 Good twist for the 50-100 grain bullets. Excellent accuracy with 60 grain bullets and up...
1/8 Good twist for the 50-80 grain bullets. Excellent accuracy with 55 grain bullets and up...
1/9 Good all around twist ratio. Will shoot well with bullets anywhere from 40-75 grains in weight...
1/10 Good twist for the 40-62 grain bullets, but favoring the lighter side a little more...
1/12 Good twist for the 40-55 grain bullets...
1/14 Good twist for the 40-45 grain bullets...

You can use the Greenhill formula to determine the proper bullet to twist ratio... It is based on the rule that the twist required in calibers equals 150 divided by the length of the bullet in calibers...

This can be simplified to:
Twist = 150 X D2/L (D2 = D squared)
Where:
D = bullet diameter in inches
L= bullet length in inches
150 = a constant

This formula had limitations, but worked well up to and in the vicinity of about 1,800 f.p.s. For higher velocities most ballistic experts suggest substituting 180 for 150 in the formula...

ETA: Bushmaster 1:9" twist barrel info using Hornady 40-75 grain ammo
Link Posted: 1/21/2006 6:28:55 AM EDT
1:9 is marginal for 75gr+. 1:7 would be much better for 75gr - 77gr ammo. Go with 1:7 if you plan to use the newer, superior 75gr+ SD ammo. Still shots 55-62gr well so no real downside.
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