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View Full Version : Max bullet weight for .40 cal 1-24"



juanvaldez
07-10-2008, 12:11 AM
Hi All

I was wondering if anyone could comment on the above. I recently purchased a pedersoli gibbs not realizing it was the slow twist version. I had already bought a 400 grain PP mold. Is their any chance it will stabilize the bullet or should I not even bother trying. The bullet weighs in a 400 grains and is 1.4" long. Only shooting out to 200 yards.

Cheers

Digital Dan
07-10-2008, 01:23 PM
It should stabilize OK based on your numbers. Weight is irrelevant, length is all that matters. Your bullet would likely stabilize with a slower twist than you have, maybe as slow as 1:36 based on a 130 constant.

leftiye
07-10-2008, 02:24 PM
Vat velocity ahre jou shooteeng it at?

juanvaldez
07-10-2008, 04:13 PM
I have not shot it yet...it should be shipping soon.Can you explain ...130 constant? What velocity should I try for? Thanks for the replies.

Cheers

Digital Dan
07-10-2008, 05:31 PM
It is an extrapolation of the Greenhill Formula. The constant is juggled to make it more applicable to different circumstances. Use 150 for higher velocities and 130 for the like of slug guns and other muzzle loaders shooting in the 1200-1400 fps range. 180 for 3000 fps or more. The formula is not applicable to round balls.

(130 x D^2) / bullet length in calibers = twist required to stabilize the bullet. Suitable for projectiles such as you describe at velocities typical for the gun.

leftiye
07-10-2008, 08:03 PM
In the older formula I'm familiar with (can't remember any name for it nor the formula itself) you had to plug in bullet weight and velocity, caliber, etc.. Slower twists can fail to stabilize at low velocity, but work well at higher velocities. Look at 1 in 16 (and 1 in 14) twists in the .22 caliber ctgs..

Digital Dan
07-10-2008, 09:59 PM
Leftiye, you almost certainly refer to the Greenhill Formula....been around since the 1800's....before smokeless powder. It's a formula what can be twisted (no pun intended) to tell you what you need to know. Of all components of the formula, weight is not part of it however. Gyroscopic stability (Sg) derives from angular momentum....the more you have, the more stable the bullet. You can have too much of a good thing though. Complex subject. Your reference to the .22s would compare the rimfire vs. hotrock centerfires from back in the day. Faster twists are more popular in .22 CFs these days since it's now popular to shoot spears instead of bullets, but for many moons the 14" twist was king. The latter has Longer bullets and requires a faster twist to impart sufficient Sg to overcome the displacement between aerodynamic center of pressure and center of gravity, that always being aft in conical bullets. Robert Rinker did a fair primer on the subject in "Understanding Firearms Ballistics", available from the likes of Amazon etc. He's not completely without fault in the book but I give him a 99% grade on veracity of content.