Twist rates

[steverjo]

There have been some excellent threads here discussing the various factors involved in twist rates for AR style rifles, with the general consensus being that the faster twist rate (1in7 as compared to 1in9) is preferable for the heavier (longer) bullets. The longer the bullet, the faster the twist rate needed for stabilization.

Why is it that calibers such as .308, or .30-'06 are not found in 1in7 or faster, but are more often in the slower twist rates such as 1in11 or 1in12? These bullets are a lot heavier/longer than the 5.56 or .223 used in AR's. Does it also have somthing to do with the diameter?

 

[SimpleIsGood229]

It's all about diameter-to-length ratios, I believe. The .308, for instance, doesn't have the case capacity to propel a projectile with the same diameter-to-length ratio as a 77 gr. .224 bullet.

 

[everallm]

A SimpleIsGood says, it is, at it's simplest, the overall geometry of the projectile round.

Some of the determining factors include,

Mass, Length, Diameter, Ogive, Base design, Internal structure, Density, Initial Impulse, Acceleration, Velocity

There are also a few particular calibre sweet spots such as the 6mm and 6.5mm range.

 

[jfdavis58]

Twist rate can be calculated by the Greenhill formula-Wiki is accurate on this subject---google is your friend. Input data: bullet length, diameter and specific gravity. Proper choice of constant relative to desired bullet velocity.

 

[Jim Watson]

There was some work done with an 8 twist in Krags, at least part of it by an old guy name of Pope.

The .30 Whisper needs an 8 twist to stabilize long heavy boattails at low velocity.

On the other hand, there was the legend that there was a run of unusually accurate 1903 barrels. Investigation showed that the rifling machine had been set up with the wrong sine bar and was turning out 11 twist barrels instead of 10. Naturally the Army set it back to mil-spec instead of the superior value.

The 19th century Greenhill empirical formula is frightfully approximate for 3000 fps spitzers, I would not buy a barrel based on it.

 

[aspade]

It's all about diameter-to-length ratios, I believe. The .308, for instance, doesn't have the case capacity to propel a projectile with the same diameter-to-length ratio as a 77 gr. .224 bullet.

A Nosler 77 gr .224 bullet has a L : D ratio of about 4.4:1. That's about the same as a 180 grain .30 caliber ballistic tip - which empirically stabilize fine in a 1:12 30-06 at about the same linear velocity.

Why? Someone who knows more physics than I do will have to answer that.

 

[SimpleIsGood229]

Thanks for that, aspade. That's interesting. I certainly cannot answer as to why, however.