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-   -   Determing barell twist (http://www.firearmstalk.com/forums/f30/determing-barell-twist-77212/)

 Tenderribbs 11-27-2012 03:55 PM

Determing barell twist

I have this formula to determine best twist rate for bullet. TR= C* D2/lL
I used 150 for C used .224 for D2 and .790 for L

I keep getting 84 for answer what am I doing wrong? I'm wanting it for a .224

 gunnut07 11-27-2012 05:10 PM

150 is that bullet weight because there is no 150 gr 224 bullet.

 The_Kid 11-27-2012 05:42 PM

I'd go here and use the revised formula.

A Calculator for Barrel Twist Rate

 Tenderribbs 11-27-2012 05:57 PM

Ok didn't know that was the weight , I used 55 for the 150 and still not coming up with a
Right answer. Here's what I'm doing

TR =55 * .224(2) divided by .790

* = multiply. Don't have a multiply sign on keyboard.

I will use the revised one and see if r helps

Thanks

 Tenderribbs 11-27-2012 06:01 PM

Another question thanks for the equation why is bullet weight not in formula?!

 The_Kid 11-27-2012 06:06 PM

Quote:
 Originally Posted by Tenderribbs (Post 1029288) Another question thanks for the equation why is bullet weight not in formula?!
Because bullet length is what is important.

 Axxe55 11-27-2012 06:11 PM

Quote:
 Originally Posted by The_Kid (Post 1029273) I'd go here and use the revised formula. A Calculator for Barrel Twist Rate
what does the SG stand for? showing a different value based on the bullet material.

 The_Kid 11-27-2012 06:15 PM

Quote:
 Originally Posted by axxe55 (Post 1029299) what does the SG stand for? showing a different value based on the bullet material.
From that page:
Quote:
 The classic equation is for solid, lead alloy bullets of specific gravity (SG) 10.9, and "when the density of the bullet is less than that of lead or the density of the resisting medium is greater than that of air, the spin should be increased as the square root of the ratio of the densities." As SG decreases, the gyroscopic inertia of the bullet decreases in proportion, and one needs to increase the spin to compensate.

 Axxe55 11-27-2012 06:23 PM

Quote:
 Originally Posted by The_Kid (Post 1029302) From that page:
thanks. didn't read that far! was thinking it had something to do density.

how do you find the SG of a particular bullet? is this information readily available?

 The_Kid 11-27-2012 09:59 PM

Quote:
 Originally Posted by axxe55 (Post 1029312) thanks. didn't read that far! was thinking it had something to do density. how do you find the SG of a particular bullet? is this information readily available?
I've seen from 10.9 to 10.7 as the benchmark for jacketed bullets.
Another example of it being used; JBM Ballistics, Drag/Twist.

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