Barrel twist

[tenure]

When building a rifle it seems everyone has a opinion on the twist rate why not use the math to settle it down and the formula is “ 180x Dia of bullet squared =total divided by overall length of bullet= Twist rate

 

[Fuj]

Or just jump on JBM and keep life simple.

Note that tipped bullets don't work perfectly in them equations when
it comes to length's. The equation does'nt account for velocities either.

 

[SSL]

Or just jump on JBM and keep life simple.

Note that tipped bullets don't work perfectly in them equations when
it comes to length's. The equation does'nt account for velocities either.

OP is listing the Greenhill Theory. In this theory velocity is figured into the equation by the 180 (for over 3,000 fps) or 150 (for under 3,000 fps). Works to some extent for rough figures, but fails to take into account certain shapes and exact velocities. There is also the Miller Theory that can be used with a bit more accuracy, but still not an absolute. Elevation, humidity barrel temperature and other things can change any of the computed values to a certain extent.

 

[jelenko]

180x Dia of bullet squared =total divided by overall length of bullet= Twist rate

What does 'total' mean?
Is that the actual formula? - if so, it says that the twist rate = 180X diameter of the bullet squared.

 

[Ranger188]

Your "math" doesn't take into account a few factors.
I'll leave it at that.

 

[SSL]

What does 'total' mean?
Is that the actual formula? - if so, it says that the twist rate = 180X diameter of the bullet squared.

Then divided by the bullet length to get twist rate. 180x Dia of bullet squared = total divided by overall length of bullet= Twist rate)
Greenhill Formula consists of C x D2 ÷ L = T where C =180 (or 150 for under 3,000 fps), D2 = bullet diameter squared (example - .244 x .244 = .05953) and L equals bullet length (example .740) Equation now reads 180 X .05953 divided by .740 = twist (1:12.06 or 1-12" twist).

As suggested, much more accurate formulas exist that include more variables and/or conditions.

 

[Ned Ludd]

Of course, then there's always the question of what twist rate barrel one can actually purchase. Depending on the manufacturer, one cannot always simply buy a barrel with whatever twist rate they would like.

 

[M-61]

Or just jump on JBM and keep life simple.

Note that tipped bullets don't work perfectly in them equations when
it comes to length's. The equation does'nt account for velocities either.

Bullet weight?
Is that in there?

 

[Dusty Stevens]

This is another case where the math doesnt always work out but experience does.

 

[Fuj]

Bullet weight?
Is that in there?

It has what you need and pretty simple to use. Go to JBM
and punch up "Stability", and fill in the blanks.

 

[4xforfun]

Is the "formula" the same for flat based and boat tail bullets? I see that Berger has two different calculators...one for FB and one for BT.