9 Twist?

[Spook]

Anyone ever try a 9 twist? I just noticed that my 220 rn loads shoot better through a 10 twist than an 8 twist. Thanks...

[LouBoyd]

Bullet stability is not just a function of velocity and twist rate. Air density plays a major roll. Shooting at higher altitude (lower pressure), during storms (lower barometric pressure), higher temperature(higher molecultar velocity) and higher humidity (lower molecular wieght) all tend to reduce the spin rate needed to stabilize a bullet.

You may find that a rifle which shoots great in warm weather can be unstable in cold weather. There is an optimal spin rate for any atmosphere, bullet, range, and velocity, and stability is not perfect at any spin rate. However, accuracy falls off more rapidly by spining a bullet too slow than too fast.

Your observation of a 10" twist barrel shooting better than an 8" doesn't surprise me at all. It might be more apparent why if you described the entire situation including the muzzle velocity, range, temperature and altitude where you are shooting.

Consider that if you could shoot in a vacuum a 1 turn in 100" barrel would stabilize a 220 gn SMK. It would be just as accurate from a smoothbore, it just might not arrive point first.

[Spook]

Lou, Shooting 1060fps (or so),220 grain RN's at 6800 ft in altitude,low 40 degree temp range, 100 yds sight in.....another funny thing is at the same time I was shooting 240SMK's through an XP pistol (14 inch barrel)with a ten twist and getting perfectly round holes at 100 yds. :confused: I always was led to believe it took an 8 twist for the 240's to stabilize.

[LouBoyd]

Lou, Shooting 1060fps (or so),220 grain RN's at 6800 ft in altitude,low 40 degree temp range, 100 yds sight in.....another funny thing is at the same time I was shooting 240SMK's through an XP pistol (14 inch barrel)with a ten twist and getting perfectly round holes at 100 yds. :confused: I always was led to believe it took an 8 twist for the 240's to stabilize.

I' suspect the 6800 foot elevaton is what's allowing you to shoot the 240 SMK's from a 10" twist barrel. . Air has a density of about .075 lb//ft^3 at sea level. and only .058 lb/ft^2 at 6800 ft. I live at 5400 feet elevation and usually get away with 2" longer twist than bullet manufacturers recommend. Stability comes from the balance of rotationaly inertia vs the force of drag trying to flip the bullet to the side. Less drag means less required spin. I'll bet at 6800 feet you get outstanding supersoinc range too. It should be about 30% further than at sea level.

It would be an interesting test if you go near the seacoast and try shooting the 240 SMKs with the 10" twist barrel. I'd expect they'll tumble.

[Spook]

Lou, Thanks for the info!