OK still little confuse but 500 S&W MAGNUM cast bullet 440 grain FN, .501 dia, .895 bullet length loaded in hand gun 1:18.75 twist with muzzle vel 1625 fps has a stability of 8.87
Same bullet loaded in my rifle (muzzleloader) 1:28 twist with muzzle vel 1150 fps has a stability of 3.485
Both data output show good stability, states anything over 1.5 is good stability, when is to much spin not good?
jbmballistics.com
Too much spin is not good when the bullet comes apart in flight. Even if it doesn’t come apart, in general the faster you spin it, the more likely an imperfection, like a void in a cast bullet, is likely to show up.
Bullet spin, RPM, is a good thing to check with higher stability numbers.
In general, bigger number is better. But....
There is also a point of diminishing return on spin. The faster a bullet spins, the higher the Stability number, the more likely the bullet is to reach its optimum BC. That number is around 2.0. So the window you are seeing is minimum required, 1.3 and more or less pointless to go past for most shooters, 2.0.
There is another, older formula you might be interested in looking at. Your bullet is closer to the model used for this equation, short, fat and flat based. The Greenhill formula. The Miller is more accurate for boattails.
Greenhill calculates twist required, based on your bullet.
Bullet diameter squared X 150 / bullet length = twist.
.501 X .501= .251
.251 X 150 = 37.65
37.65 / .895 = 42.07
Plug your bullet into JBM, .501”, 440 grains, 1/42 twist, 1200 fps. It spits out 1.509 for a stability factor.
The original 50-70 government rifles used a 1/42 twist, one of my molds is .515”D .950”L, 440 grains. Roughly 1200 fps from a 22” barrel carbine.
Shoots pretty well for a 150 year old rifle, bullets are stable at 300 yards.
At least I think they are. It’s hard to tell on paper when the vertical drop is faster than the forward progress.
I don’t think either of your barrels will over spin your bullets and cause accuracy issues. Your fastest load, in your fastest twist barrel is spitting them out around 65,000 rpm. That’s about half what most people might consider the top end of a good cast bullet.
For what it’s worth, I shoot a lot of cast bullets that are either way too low or high on the stability charts. Within a couple hundred yards, you can have very good results outside of what all the formulas recommend. Stretch it out past 500, and it really can make a difference.
I think that the biggest problem you might have with a big heavy bullet, in a fast twist barrel, might just be the torque generated.
