Determining optimal rifle twist using the Greenhill formula.

[sharpie443]

I got a question from someone on youtube asking why rifling and bullets in general haven't changed much over the years. His message boiled down to him wanting to know why we don't try to make better rifling to improve accuracy. Obviously there are a number of things wrong with his question but I don't think a lot of new shooters understand rifling or how we determine what twist will gyroscopically stabilize a round.

So I made a video explaining the Greenhill formula and how we determine what twist to use with different projectiles. I also talked about needing a faster twist with subsonic and things like that.

I figured someone here might be interested in it. The video involves math so I’m not sure how popular it will be.

http://www.youtube.com/watch?v=4WKztzgcnQw&list=UU_i0O7tE8cUH0CTk3VqtM6w&index=1&feature=plcp

 

[Scorch]

His message boiled down to him wanting to know why we don't try to make better rifling to improve accuracy.

But we do. There are many, many types of rifling that have been developed and marketed, most recently the 5R rifling and polygonal rifling. Look at historical firearms and you will see many different types.

 

[sharpie443]

O I know but he didn't even understand the basic phisics of behind it. I wasn't going to go into that king of thing. The point was that we know what twist will stabslize a round it's just that we have developed new ways to impart the same twist. There are better ways of determining the twist as well. The Greenhill formula is just easy to explane. It was originally for use in rifled artillery.

 

[Bart B.]

The interesting thing about the Greenhill formulas is while popular, top level competitive shooters use different twists for their bullets than what Greenhill calculates.

In 1879, Greenhill developed a rule of thumb for calculating the optimal twist rate for lead-core bullets in small arms. This shortcut uses the bullet's length, needing no allowances for weight or nose shape. Greenhill applied this theory to account for the steadiness of flight conferred upon an elongated projectile by rifling.

With modern bullets at the speed they leave at, it's not all that useful. The slowest spin rate that keeps a bullet stabilized all the way to the target is the best one to use. Any faster and the normal, slight unbalance of bullets (they're all unbalanced to some tiny amount) causes them to wobble too much increasing drag proportionally to their rpm's. Unbalanced bullets also jump off the bore axis as the leave the muzzle due to centrifugal forces and take a more spiraled flight to the target.