rate of twist
- Thread starter sirley
- Start date
There is a formula called the Greenhill formula which calculates the maximum length bullet that will be stabilized by a given twist. Formula is 150 diameter squared divided by length equals twist needed or divided by twist equals length stabilized units in inches. Most people would make the twist rate 1" faster than the formula. It allows you to calculate the longest bullet your rifling will stabilize.
cheers mooncoon
cheers mooncoon
Ok, now is that 150 'times' the diameter of the bullet ? Then squared ?
And length I would guess is the bearing length of the bullet ?
But I'm stuck on the 'make it 1" faster' comment . .
WithoutWarning, that formula is OK, but there are other variables to be considered as well, like flat base or boat tail, lead core or pure copper etc.
That 1-11 twist will stabilise anything right up to 200 grain boat tails. A 1-12 twist will work fine as well in most cases.
When the 1-10 twist rate was first established, almost all of the bullets were very long 220 grains and over round nose design at lower velocities than we commonly shoot today. The 1-10 twist was a bit overkill even then, but it's better to err on the side of to much twist than not enough. To much twist will very slightly elevate pressures and very slightly lower velocities, not enough to notice though. Benchrest shooters have found that a bullet that is just stabilised will give the best accuracy. bearhunter
That 1-11 twist will stabilise anything right up to 200 grain boat tails. A 1-12 twist will work fine as well in most cases.
When the 1-10 twist rate was first established, almost all of the bullets were very long 220 grains and over round nose design at lower velocities than we commonly shoot today. The 1-10 twist was a bit overkill even then, but it's better to err on the side of to much twist than not enough. To much twist will very slightly elevate pressures and very slightly lower velocities, not enough to notice though. Benchrest shooters have found that a bullet that is just stabilised will give the best accuracy. bearhunter
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also now with faster rifles it is suggested the 150 be replaced with 180.
this still has a lot of assumptions. I believe the load from disk software uses multiple complex formulas for various bullet speeds. you should be alright in your rifle though.
this still has a lot of assumptions. I believe the load from disk software uses multiple complex formulas for various bullet speeds. you should be alright in your rifle though.
the diameter only is squared. The length is total length. Increased velocity on a borderline twist will allow a slightly longer bullet but not enough so to worry about it, in my opinion.
Greenhill by the way was an american artillery officer in the 1870's who developed the formula for artillery projectiles but it works also for rifle bullets.
Relative to Bearhunter's comment; the problem with using bullet weight is that a 50 gr boat tail is slightly longer than a 50 grain flat base and if the twist is borderline on the flat base, it will not stabilize the boat tail. The problem is not the shape as such but the slightly increased length.
There are also constants for projectiles of unusual density but partly I can't remember them and partly it is unlikely that you will be shooting depleted uranium or aluminum bullets.
cheers mooncoon
Greenhill by the way was an american artillery officer in the 1870's who developed the formula for artillery projectiles but it works also for rifle bullets.
Relative to Bearhunter's comment; the problem with using bullet weight is that a 50 gr boat tail is slightly longer than a 50 grain flat base and if the twist is borderline on the flat base, it will not stabilize the boat tail. The problem is not the shape as such but the slightly increased length.
There are also constants for projectiles of unusual density but partly I can't remember them and partly it is unlikely that you will be shooting depleted uranium or aluminum bullets.
cheers mooncoon
I Have a 30-338 with a 1-12" twist. I have shot it right out to 1000 yards (for fun) and have taken game out to 450+ with it. It stabilizes any 200 grain bullet I have ever shot in it without problems. I start the 200 at 2980 FPS, and the 180 at 3150. Your 1-11" should be fine for any "hunting" bullet up to 200 grains, including Boattails. Regards, Eagleye.
Every Husqvarna rifle made in both 308 and 30-06 has a 1:12 twist and they have no problem at all stabilizing the heavy bullets.
Your 300 WSM at 300 fps higher velocity will do fine with the 11 twist barrel.
Ted
Your 300 WSM at 300 fps higher velocity will do fine with the 11 twist barrel.
Ted
