Some useful info + the formula to determine the twist for bu

[Lee 9]

Some useful values:

1 grain = 0.037 dram
1 pound = 7000 grains
1 pound = 16 ounces
1 ounce = 437.5 grains
1 ounce = 16 drams
1 dram = 27.344 grains
Density of lead is 11.34 gram/cc
1 gram =15.4324 grains
1 inch = 2.54 centimeters
1 meter = 39.37 inches, 3.28 feet and 1.09 yards
pi = 3.14159
Volume of a sphere = 4/3pi R3 (4/3 x pi x radius of sphere cubed)
Radius of a sphere = cube root of (3Volume/4 pi)
Volume x density = weight

To find Twist needed for a bullet
150 x diameter squared divided by bullet length = required spin
Example for a .45 caliber bullet .60 inches long:
150 x .45 x .45 divided by .60 = 50.6 inches
So, for the example bullet, a spin rate of 1:50.6 or faster is required

 

[sabotloader]

Lee

Thanks for posting this information over here also.

I would also like to pos the follow-up post you made that goes along with this information...

Thank you Mike, I will pass you back a few figures but remember we are speaking of what is ideal not absolutes.
It is relatively easy to get away with some over stabilization but if a bullet is under stabilized it may stray or tumble so take into consideration that the figures I am putting down are the ideal maximum length figured for 1800 fps and speed increases the maximum length so does a faster twist.
For a .458 diameter bullet in a 1/28 twist a length of 1.1 inch [ if I did not make any mistakes.]
For a .450 diameter bullet in a 1/28 twist a length of 1.05 inch .
For a .400 diameter bullet in a 1/28 twist a length of .850 inch.
Now remember this is ideal max length and that shorter bullets will shoot fine also that the weight is not a consideration and the this is based on 1800 fps. By pushing a bullet faster it add spin as well as velocity.
Just for fun and comparison
For a 1/24 twist at 1800 fps .458=1.30
.450=1.25
.400=1.00
Have fun Lee

This response was generated as result of one my other posts. I supplied Lee with the information he requested and he computed the information for me resulting from this picture.

Again thanks Lee

 

[minst7877]

A question for you to mull over. Do you include just the bullet length or should you also include the sabot as it makes the overall length longer?

DC

 

[Banerbird]

Good info guys. I always wondered (other than trial and error) how to figure out if a bullet would work with your gun's twist rate.

As far as including the sabot in the overall length... My guess would be no. The sabot separates from the bullet almost immediately leaving just the bullet that needs stabalized down range.