Twist Rate for A-Max

[300magman]

Can anyone direct me to a formula or program that calculates the necessary twist rate to stabilize various bullets?

I am interested in trying the 75gr A-Max in a 22-250 AI and also the 208gr, or the rumoured new 225gr A-Max, in the new 300 Norma Mag (improved).
I want to be sure to use a barrels with twist rates fast enough to stabilize these bullets at sea level, but I don't want to have overly fast twist barrels as I may try some lighter bullets as well.

[kraigwy]

The recommended twist for the 75 Gn A-Max would be 1:7 (actual 6.78)

The recommended twist for the 208 grn A-Max would be 1:10 (actual 9.29)

Lenght of bullet divided by cal.

Take that number and divide it into 150

Multiply that number by the cal.

So lets take the 75 A-Max which is 1.110 in. long.

1.110/.224 = 4.955
150/4.995 = 30.03
30.03 X .224 = 6.78 or 1:7

Error on the side of faster twist, You can shoot 52 gr A-max in a 1:7 with no problems but you can't go the other way.

[maggys drawers]

http://www.riflebarrels.com/products...wist_rates.htm

The link is to Lilja barrels. At the bottom of the page is a projectile weight/twist rate chart.

[Unclenick]

Actual direct calculations are quite complex, so you use an approximation. The Greenhill formula that Kraig illustrated is the best known one. It was originally intended for artillery shells, and Greenhill experimented with prolate spheroid (egg shaped) projectiles of specific gravity (S.G.) 10.9 to develop the formula. By sheer felicity, it turns out it isn't too bad for many highpower rifle rounds. Within the limits of the density of the bullet being similar (S.G.=10.7 is a common approximation for jacketed lead core bullets), and the velocity being under 2800 fps, it will usually get with 10% or so of where you would be ideally. (Search the web and you will find 150 gets replaced by higher numbers for higher velocities, and a density adjustment wherein you multiply the result by the square root of your bullet's density divided by 10.9. That's for use with copper solids, for example.) Greenhill assumes standard meteorological conditions (29.53 in Hg, 59°F, 78% R.H.)

Bullet weight vs. twist tables make the same assumptions about bullet density and metro conditions, but have a further assumption built in that the bullet shapes the shooters will use are all about the same. The problem is that bullet length has a more significant effect on on stability than weight, being the lever arm by which frontal air pressure tries to flip the bullet center of gravity around. Thus, a flat base, round nose bullet of a given weight needs less spin than a VLD the same weight to achieve the same degree of stability. The Greenhill formula with adjustments is better than weight tables because it takes length into account. Because the A-max is longer than the average bullet, the Lilja table significantly underestimates the necessary twist for it, leaving it too close to the edge of stability. It will need several hundred yards to go to slip with the 9" twist rate.

Don Miller devised a fully modified version of the Greenhill formula that usually works pretty well and that takes length, weight, velocity, and atmospheric conditions into account. You enter the bullet length, weight, and velocity and weather conditions of your choice. You also enter a barrel twist rate you want to try. It's output is a gyroscopic stability factor and you can make that any number you choose. The normal recommendation for match shooting is 1.4 to 1.7, with 1.4 to 1.5 being preferred over the higher end of the range. For hunting accuracy, 1.3 to 3.0 is generally acceptable (I got that hunting range from a Sierra tech).

For example, with the 75 grain Amax, the Lilja table says to use a 9" twist. Kraig showed a 6.8" twist being recommended by Greenhill, unadjusted. If I use Don Miller's fully adjusted version of the Greenhill formula, then assume a velocity of about 3400 fps in the .22-250 AI under standard metro conditions, I get a 7.8" twist for a stability factor of 1.4 and a 7.5" twist for a stability factor of 1.5. If I go more broadly with 1.4 to 1.7, then the range is 7.8" to 7.1" twists, the latter number being very close to Kraig's actual recommendation, which recognized Greenhill erring on the fast side of twist.

If I were buying the barrel with any twist I wanted, I would probably choose 7¾". My AR has a 7½" twist which shoots the single-loaded A-max just fine, but the velocity is much lower than yours will be. Allowing for the greater velocity you have, the 7¾" should do fine.

The Miller version of the Greenhill formula is available to use free on the JBM ballistics site, here.

[kraigwy]

Here is an example regarding my belief one should error on the side of faster twist.

[Unclenick]

No doubt, short of making bullets strip their cores or fly apart in mid air, too much spin is better than too little. But too much also comes at an accuracy cost. The M16A1 vs the M16A2 is a perfect example. They have a 12" and a 7" twist respectively. The Europeans figured out best accuracy with SS109 ammunition was achieved with a 9" twist when they developed it, which I expect is why 9" twists are popular in non-match civilian version AR's. The 7" twist was adopted only because they also had to stabilize the lighter weight tracer version of the ammunition, and decided that for military accuracy the excessive spin was acceptable.

The idea that there is an optimal gyroscopic stability factor, whether one authority puts it at 1.4 or 1.5, is that the choice is a compromise between the need for spin for stability and for putting yaw to sleep over a reasonable distance, and the opposing need to minimize wobble in flight and to avoid increasing lateral jump at muzzle exit. Both wobble and muzzle jump open groups up. Both depend on some kind of mass asymmetry being present in the bullet to occur, though, so they're a bigger problem with lower precision bullets and less troublesome with match bullets.

The reason short range benchrest shooters use stubby flat base bullets is the shorter shape lets them get away with using slower twists that produce even less wobble and muzzle jump than normal. Part of the high accuracy game.

Choosing an optimal twist is good for highest accuracy. It's less flexible about conditions though. If, like the military, you need to be able to operate in all environments, then you have to keep in mind that a twist that is optimal in summer can be too slow to keep a bullet stable in extreme cold. You also have to worry about stabilizing a fairly wide range of projectiles. Like so many other choices with firearms, you have to know your purpose.

[cookhj]

1-10 works best for the 208, but there are guys on snipershide who have been getting good accuracy with a 1-11.25 and 1-12. it might not work in all rifles though.

[Unclenick]

Cookj,

That's .30 cal. The OP is using a 22-250 AI.

[kraigwy]

He did ask about the 225 gr A-max also. I couldn't;t find info on it thats why I gave the formula.

[Unclenick]

You're right. I brain blanked on that. Apologies to Mr. Cook.

If you call Hornady, they are open with information on length, assuming they've finalized the design. If the design uses the successful nose and tail form of the 208, which seems likely, then assuming a specific gravity of 10.7 in the bearing surface area, then the new design would come out at 1.614" long and with a BC 8.3% higher.

The JBM calculator thinks those changes running at 2850 fps, will still be about best with the old standard military 10" twist. But I'm making a lot of assumptions there. I'd still call Hornady.