Rate of Twist and Bullet Stability

[dverna]

Typically, a higher rate of twist allows a heavier bullet to achieve improved accuracy. This is why the .243 surpassed the .244 and why AR15's using heavier bullets have a 1-7 or 1-8 rate of twist vs 1-12 for "normal" .223's

It makes sense that a slower moving bullet will require a faster rate of twist to achieve the same rotational speed as bullet driven at higher velocity.

What I do not understand is why calibers like the 38-55 and 45-70 (very slow bullets) have a rate of twist of about 1-18. It would seem these slower bullets would need a faster rate of twist to stabilize bullets.

Can anyone explain this?

Thanks,

Don

[felix]

Inertia. The more inertia a projectile has, the more it resists extraneous forces. ... felix

Notice how slowly Space Station #9 rotates. Lots of mass.

Notice how a satellite does not rotate at all. It's inertia is derived from rotating around the earth.

[HORNET]

Look at the Length/Diameter ratios. When you get to something like the 6.5 Cruise Missile (170 grains), it's about 5 calibers long (guessing, since I don't have one). Even the real heavies in .38/55 only get to about 3 calibers long (again, a guess, since I don't have one handy). Please notice that the twist rates on some of the big bores are getting faster as the heavier boolits with higher ballistic coefficients gain popularity. Many of the BPCR boys are going to twists in the 14" range.

[mooman76]

The larger the bullet in diameter, the less twist it needs to stablize it. Basically what they said.

[jimb16]

Round ball has a 1:1 ratio of length to width. It will stabilize at a twist rate of 1:72!

[44man]

My 45-70 revolver will out shoot every rifle to 500 meters with a whole range of boolits from 300 gr to 485 gr. It has a 1 in 14" twist.
I never tried heavier boolits, no need.
My 1 in 18" Browning is a bear to find long range accuracy with and a 1 in 20" will drive you nuts.
Shorter, lighter boolits can use a slower twist because you can shoot them faster but you will run into a wall with long, heavy boolits so you need to spin them up.
I will always go to the faster twist because you can down load to fit but you can't speed up a heavy boolit enough for a slow twist.
The round ball needs a slow twist but most from .45 to .54 do better with 1 in 60" to 1 in 66". Anything slower will cut long range accuracy.
Look at the Marlin .44 mag with the 1 in 38" twist! What a hunk of useless steel. I would prefer a 1 in 20" and a 1 in 18" in the revolvers.
BFR revolvers are the ONLY guns that use a faster twist and they are the most accurate I have ever shot, had several 100 yard groups down to 5/8" and many, many at 1" or less. If I can see a 1" target at 100, I can hit it.
Slow twists are short range guns.

[dverna]

Thank you gentlemen.

I was able to find a formula that determines the rate of twist need to stabilize a bullet.

http://en.wikipedia.org/wiki/Twist_rate#Twist_rate

It clearly shows the relationship of bullet length and diameter.

44man,
I also have one of those "hunks of useless steel" Marlins in 44 Mag. I wind up using 200 gr. bullets to get 3" groups at 100 yards. I don't hunt so I have not tried anything heavier but you are probably correct that the twist is too slow.

Based on the formula (see above link) the max bullet length that can be stylized in a .429 bore with a 1-38 twist is less than 3/4".

Don

[44man]

Thank you gentlemen.

I was able to find a formula that determines the rate of twist need to stabilize a bullet.

http://en.wikipedia.org/wiki/Twist_rate#Twist_rate

It clearly shows the relationship of bullet length and diameter.

44man,
I also have one of those "hunks of useless steel" Marlins in 44 Mag. I wind up using 200 gr. bullets to get 3" groups at 100 yards. I don't hunt so I have not tried anything heavier but you are probably correct that the twist is too slow.

Based on the formula (see above link) the max bullet length that can be stylized in a .429 bore with a 1-38 twist is less than 3/4".

Don

I thought about round balls but sold the gun instead!

[ironhead7544]

The 38/36 inch twist was an attempt to get good long range accuracy from a 200 gr bullet in front of 40 grains of black powder. My Marlin Cowboy 44 Mag shot excellent groups with a 200 gr RNFP at that same velocity.

[missionary5155]

Round ball has a 1:1 ratio of length to width. It will stabilize at a twist rate of 1:72!

With RB once you get to the larger sizes of caliber .65 and above even slower twists are enough. Seems a .72 will stabalize with as little as 1 : 90.

[44man]

With RB once you get to the larger sizes of caliber .65 and above even slower twists are enough. Seems a .72 will stabalize with as little as 1 : 90.

I don't know what the twist is for an original trap door or rolling block or for that matter a .58 musket but they worked.
I have never had a problem with cut rifling the right depth but some modern muzzle loaders are broached and even with a proper twist, they are hard to get accuracy from. The TC Hawken is one example. The original was great but they changed something with the rifling. Rifling does not appear as deep any more and you need to use a smaller ball.
The Lyman GP rifle shoots better then any other factory round ball gun.
My Browning BPCR has 8 lands and grooves that are too shallow.
The muzzle loaders I built all have deep cut rifling and shoot great at 200 yards.
Marlin changed to Ballard but all they did was reduce lands and grooves keeping the same .003" deep rifling of the micro groove. The micro groove actually shoots better.
So maybe twist alone is not enough but how the barrel is made should be considered. A round ball gun should not have a slippery bore, if anyone ever worked with a chrome plated abortion they would see.
Old timers would shoot a bore smooth and etch it over night with urine. I have used vinegar to bring back accuracy. No luck at all with a newer TC.
Grooves on a round ball gun should be .010" deep.
Am I off base here? Or should a boolit shooter have deeper rifling? Should shorter barrels have a faster twist? I have always wondered what a .44 revolver would do with a 1 in 18" to 1 in 19" twist.

[calkar]

I believe, like Hornet mentioned, that there is some length to dia. law in bullet design that the cruise missile violates. A length that is beyond the practical ability to stabilize.