Handgun Twist Rate

[rodwha]

A ROA has a 1:16" twist. How do I figure out the longest projectile it will stabilize?

If the Greenhill formula figures 180 for high velocity rifles, 150 for muzzleloading rifle, would it be fair or accurate enough to use 120 or so for the figure assuming the velocity would be in the mid to upper 800 ft/sec range?

 

[Jim Watson]

I don't know about Greenhill or how to fudge its constant to give the answer you want, but working in Berger's twist calculator, a 507 gr .45-70 bullet, 1.35" long assumed sized down to .454" and shot at 800 fps would have a Miller stability factor of 2.95 where 1.5 is considered "comfortable stability."

 

[rodwha]

I'm wanting Tom at Accurate Molds to design me a heavy pistol bullet for hunting things that may want retribution. Penetration is paramount, but weight/length takes up powder capacity, and there needs to be a balance.

I keep looking at .70" as a maximum OAL and a weight of around 270-275 grns.

Kaido's 255 grn FN has an OAL of .635". Adjusting the calipers out to .70" looks like that would be close to the powder space given up by reducing the powder charge ~16% as Mike Beliveau had, which would still allow the 25 grn 3F charge he used.

Kaido's 240 grn FN has an OAL of .616" and I can easily get 35 grns of 3F T7 behind it.

 

[Eb1]

http://kwk.us/twist.html

Bullet length and velocity along with material determined twist rate more than weight, but heavier bullets are generally longer.

 

[rodwha]

According to everything I keep seeing is that a ROA with a 1:16" twist should be able to stabilize a very long bullet.

Something else I hadn't accounted for with Mr. Beliveau's load was that he calculated minimal compression. This isn't necessary with non cartridge guns as Hodgdon's site clearly states minimal compression is only for cartridges, whereas the rest need "firm" compression. I've emailed them asking what exactly "firmly" means with no response.

If I exclude his powder reduction of 16% it brings the charge up to 30 grns.

 

[Eb1]

This is a rifle barrel, but I am getting a 475 super mag made, and the barrel twist is 1 in 18. It will handle 300+ grain .430 bullets easily. They will be moving around 1800 to 2000 fps.