Strykervet

July 15, 2011, 09:04 PM

The "easy" optimum twist rate equation for a general estimate is as follows:

Twist=((CD^2)/L)(Sqrt.(SG/10.9)), another way of writing this is: Twist=(CxDxD)/L x the Square root of SG/10.9. C=150, but it equals about 180 for bullets travelling faster than 2800fps. D=diameter of bullet in inches. L=bullet length in inches. SG=specific gravity; for lead core bullets, use SG=10.9, which cancels out the second half of the equation leaving Twist=(CxDxD)/L. For copper bullets, like Barnes, use SG=8.96. This equation may look difficult, but it really isn't, you just plug in the numbers. Here is an example:

The 120gr. SMK is 1.305" long according to Sierra, 6.5mm=.264". The bullet is going about 2600-2700fps. So optimum twist for this bullet is: Twist=(150x.264^2)/1.305=8.011. The average (based on my calculations) for 100-123gr. bullets is about 9.

You can use this equation for any round. You can also find better values for "C" if you look around, search for twist rate equations, which will give you a better estimate. This is really valuable if you intend on firing a specific bullet out of a custom rifle --with this equation, you can have the perfect barrel built. Most are compromises of some sort in order to allow the shooter the option of using many bullet designs. With AR barrels, you can get many different twist rates, each suited to a certain range of bullet weights/lenghts. If you know which bullets you intend on using, this equation can make barrel selection much easier and much more suited to the application. If you already have a barrel and load for it, then algebraically manipulating this equation can give you the optimum bullet lengths to use as well.

Enough with the geek talk. I just thought it would be good info for those that haven't heard it before. It also pertains to the following:

So I got one of the 20" Satern AR Grendel barrels, 1-8.750, the last one since they are discontinuing them. Probably a good compromise for several bullet weights based on another calculation similar to mine. Anyway, for those of you that load and reload Grendel, based on your own data, what works best for you as far as bullet weight and accuracy, particularly long range? What twist do you use?

Any luck with Varget? What about H322 or others? Is the Lapua brass worth the price over the Hornady?

I'm interested in your load data also, but not that I want to duplicate it, rather I would like to have an idea of what others are using. I plan on working up my own loads of course, but I would like to narrow down suitable components that are working for others.

Thanks you!

Twist=((CD^2)/L)(Sqrt.(SG/10.9)), another way of writing this is: Twist=(CxDxD)/L x the Square root of SG/10.9. C=150, but it equals about 180 for bullets travelling faster than 2800fps. D=diameter of bullet in inches. L=bullet length in inches. SG=specific gravity; for lead core bullets, use SG=10.9, which cancels out the second half of the equation leaving Twist=(CxDxD)/L. For copper bullets, like Barnes, use SG=8.96. This equation may look difficult, but it really isn't, you just plug in the numbers. Here is an example:

The 120gr. SMK is 1.305" long according to Sierra, 6.5mm=.264". The bullet is going about 2600-2700fps. So optimum twist for this bullet is: Twist=(150x.264^2)/1.305=8.011. The average (based on my calculations) for 100-123gr. bullets is about 9.

You can use this equation for any round. You can also find better values for "C" if you look around, search for twist rate equations, which will give you a better estimate. This is really valuable if you intend on firing a specific bullet out of a custom rifle --with this equation, you can have the perfect barrel built. Most are compromises of some sort in order to allow the shooter the option of using many bullet designs. With AR barrels, you can get many different twist rates, each suited to a certain range of bullet weights/lenghts. If you know which bullets you intend on using, this equation can make barrel selection much easier and much more suited to the application. If you already have a barrel and load for it, then algebraically manipulating this equation can give you the optimum bullet lengths to use as well.

Enough with the geek talk. I just thought it would be good info for those that haven't heard it before. It also pertains to the following:

So I got one of the 20" Satern AR Grendel barrels, 1-8.750, the last one since they are discontinuing them. Probably a good compromise for several bullet weights based on another calculation similar to mine. Anyway, for those of you that load and reload Grendel, based on your own data, what works best for you as far as bullet weight and accuracy, particularly long range? What twist do you use?

Any luck with Varget? What about H322 or others? Is the Lapua brass worth the price over the Hornady?

I'm interested in your load data also, but not that I want to duplicate it, rather I would like to have an idea of what others are using. I plan on working up my own loads of course, but I would like to narrow down suitable components that are working for others.

Thanks you!