The Twist rate formula. (From 30 cal Lapua post)

[Jagerdad]

In my initial post of 30 cal Lapua Cutting Edge more data was emailed to me from apparently another forum. I've added this post to include the data's origins. I failed at writing down the formula precisely. It should read 150xDxD (Over) Length = Twist.
The below post had more data but now instead of being smart and just loading I'm contemplating if (how) velocities may also influence this formula?
I understand that velocities effect a bullets BC (Ballistic Coefficient) so does velocities also play a role in twist rates?
I save money in components by playing these mind games because I get nothing done.
I don't have a Mathematician next door but I think the twist formula could be expanded in deravations to include velocities.? If you know of a person who speaks Math as a second language please forward this to such person and let me (Us) know the findings Please.

http://www.perfectunion.com/vb/ruger...et-weight.html

http://www.z-hat.com/twistrate.htm

150 X D X D = Twist
—————
Length

[kraigwy]

I simply use the Greenhill formula. It seems to work for me. I know there are others out there who like to put a lot of other informant ion in the game, but personally (and this is just my opinion) I don't think its necessary.

If you check the manufactor's twist recommendations for a given bullet, then put the measurements of that bullet into the Greenhill Formula, you'll find they are pretty dern close.

Having said that, if you have to error, then error on the side of a faster twist. A faster twist will shoot lighter bullets better then a slow twist will shoot heavy bullets.

[amamnn]

I go another step further in simplification. When I buy a barrel I know what bullet diameter I am going to shoot and what my application is. I then look on the barrel maker's chart--my fave is Shilen --and select the twist rate for the weight (length) of bullet I will shoot. An example is a 6mm barrel with a 1/14 twist for a 68 gr. short range BR rifle is pretty common. I know I will not be shooting anything but 60 something grain bullets for that application.

When operating in reverse, and trying to match a bullet to an existing barrel, I cheat and look at Walt Berger's website where he lists all the recommended twists for his different bullets..........save a burning up a few brain cells and I need what I have left................................

[FrankenMauser]

The below post had more data but now instead of being smart and just loading I'm contemplating if (how) velocities may also influence this formula?
I understand that velocities effect a bullets BC (Ballistic Coefficient) so does velocities also play a role in twist rates?

Yes, they do.

For example:
A 1:14" twist .22 Hornet will almost never stabilize 50-60 grain bullets with its max velocities of 2100-2600 fps.
Yet, a 1:14" twist .220 Swift will always stabilize 50 grain bullets just fine with 3,800 fps, and eats 60 grain bullets at 3,600 fps without issue.

Velocity can make up for a slow twist rate. (It's all about the bullet RPM.)
In some cases, the slow twist rate is required, due to high velocities. A good example here, again, is the .220 Swift.
A 1:14" twist .220 Swift can shoot brand X 40 grain HPs all day long, at 4,200 fps.
But... If you throw a "competition barrel" on there with a 1:7", 1:8", or 1:9" twist (designed for 70 gr long range stuff), the 40 grain HPs will tear themselves apart when they leave the muzzle. Again, it's all about RPM. The centrifugal force the bullet experience, due to the incredibly high RPM, makes it "explode".

For more info, and more detailed explanations, search the rifle forum and reloading forum.
Here are two threads of interest:
http://thefiringline.com/forums/showthread.php?t=318647
http://thefiringline.com/forums/showthread.php?t=344719

[Ideal Tool]

Hello, Frankenmauser. Concerning that 1-14" twist in a Hornet..almost never stabilizing 50-60gr. bullets at 2100-2600fps. I must take exception. My Shilen barreled Sharps Borchardt Hornet groups well under 1/2" at 100yds. using 55gr. CAST LEAD bullets..either plain-base or gas-check, at velocities well under those listed above.

[FrankenMauser]

Hello, Frankenmauser. Concerning that 1-14" twist in a Hornet..almost never stabilizing 50-60gr. bullets at 2100-2600fps. I must take exception. My Shilen barreled Sharps Borchardt Hornet groups well under 1/2" at 100yds. using 55gr. CAST LEAD bullets..either plain-base or gas-check, at velocities well under those listed above.

There are always exceptions.

However, you're also shooting a very dense (short) bullet, if it's cast. Bullet length has far more effect on stabilization, than actual weight does. It's just simpler to give examples by bullet weight, rather than bullet length; since that is what most shooters can easily identify with (and length generally increases fairly linearly with weight increases).


** And most shooters generally identify with jacketed bullets, before cast bullets. So, again, giving examples based on jacketed bullets is easier. Though, I admit - Not all inclusive.

[mc223]

Try this calculator

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi