A gunsmith friend of mine and I talked about the different twist rates and their effects on velocity. He has been told that they can make a noticable difference. He is building my 500 A2 so we are discussing the twists available.

Any opinions?

Mike

Mike the challenge with twist rates is to use the least amount of twist necessary to stabilize the bullet. To calculate twist rates you need the length of the bullet and the intended velocity range. There are many good Internet sites that will allow you to enter the specifics on the bullet measurements, velocity, and environmental conditions in order to calculate twist rates.

Most likely you will be in the 1:10, eight groove rifling area. Unless you are looking at a cut rifle barrel and considering light for caliber bullets, you will have very little leeway for anything else.

One thing to consider is the number of grooves. Take the .300 WSM for instance. It appears that in an effort to increase the velocity potential, Winchester opted for 4 groove rifling. It will take some time for that to fully soak into the industry, but I am sure that it will lead to some changes on rifling barrels.

Mike,

The Greenhill Formula will help:

http://www.recguns.com/VIIIB6.html

The Formula predicts how much twist you will need for a given *length* of bullet.

The old English makers used some pretty slow twists in their big-bore double guns as compared to modern magazine rifle barrel makers.

jim

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"if you are to busy to
hunt, you are too busy."

I posted a question on the forum inquiring as to the best twist for a 338/06 & was told that faster twist gives deeper penetration.

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NRA Life member

[QUOTE]Originally posted by Bear Claw
HI Bear Claw,
I was trying some experiments when we were discusing bullet stabilization and penetration here on the sight. The reason was some thought the tests that I did for penetration only weren't correct because the plywood etc was shot at such close range not giving the bullet time to go to sleep or stabilize. So I tried loading my rifle to different velocities so the impact velocity at the target would be the same 2400 fps. It was shot from point blank, 25 yards and three shots from 50 yards. The biggest variation was the muzzle velocities so the impact velocity would remain at 2400 which would mean that the rotational velocity would be higher from the shots at 50 yards than at point blank. End result was that all the bullets were recovered in the same piece of plywood at 34 inches. The results are on my website at www.470mbogo.com under the Comparing the Big Bores portion.
470 Mbogo

Bear Claw - Who told you this?

How can a bullet penetrate better coming from a 1:10 VS a 1:12 twist all things being equal? In a elephant sized animal, the relative rotational variance between the two bullets is inconsequential - a partial rotation at best. Bullets do not act like drill bits - unfortunately.

I don't remember who it was Zero. Check the threads on the 338/06.

Doug

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NRA Life member

BC - Search high and low for the thread - no joy. I am probably telling you something you already know, however.... Twist only serves one purpose - to stabilize a bullet. Also known as Static Stability. Too little and you have wobble; too much (over stabilization) and the bullet keyholes on paper or does not track straight through game. In an over stabilized situation, the bullet hits the target at the same angle that it left the muzzle. It does not follow the trajectory curve. Also known as Tractability. While this is not a big deal at short yardage, it quickly becomes an issue at longer ranges i.e. higher muzzle angles to account for bullet drop.

While bullets spin at relatively high RPM rates (rotational velocity), when you take velocity into account, they spin relatively slow over a given distance - like one full rotation for every foot traveled (i.e. 1:12 twist). As a result, rotational velocity has little impact on penetration assuming that the bullet is properly stabilized and enters the animal in a proper attitude. Therefore, the �drill bit� effect is fictitious.

Zero this is the post I got that information at
http://www.serveroptions.com/ubb/Forum2/HTML/000968.html
Check out Gerards entrys.

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NRA Life

[This message has been edited by Bear Claw (edited 03-12-2002).]

BC - Thanks for the link. Everything in the post matches the physics of twist and dynamic stability. The three factors you need in order to calculate twist is the length of the bullet, velocity range, and to a lesser extent environmental factors. Change any one variable much and you will effect the twist rate necessary to achieve dynamic stability.

As HunterJim pointed out the Greenhill Formula is used to calculate twist. Believe it or not, the threshold necessary to achieve perfect dynamic stability is fairly narrow. Remember too little and you get wobble, too much and the bullet is over-stabilized.

Gerard pointed out that the Lapua rifle was specifically designed as a long range people eliminator. The bullet for sniper use was was selected first, and they then calculated that a 1:12 barrel was needed. For hunting applications if you decided to use a 250gr or a 150 gr bullet, a 1:12 twist may be too little or too much respectively.

As bullet length (weight) increases, the necessary twist rate decreases assuming all other variable remain constant. i.e. 1:12 with a short-for-caliber bullet, to 1:9 with a long-for-caliber bullet.

As I stated earlier, most of this is unimportant for distances out to 150 yards. However, for long range shooting, the dynamic stabilization window becomes quite narrow. In addition, bullets which may be stable at 100 yards may become unstable at 1000 yards due to a loss in velocity. This is one of the challenges of shooting LR.

This if how goofy it can become - For example, in my 6mm LR target/varmint guns the following holds true:

For a 60gr bullet the twist rate should be 1:14
For a 65gr bullet the twist rate should be 1:13
For a 75gr bullet the twist rate should be 1:12
For a 90gr bullet the twist rate should be 1:10
For a 105gr bullet the twist rate should be 1:9

For my last 6mm LR gun, I spent a lot of time considering the bullet that I wanted to shoot. I needed a high BC so I stayed in the heavy for caliber range. Accordingly, I settled on a 1:10 twist. I have three 6mm guns a 1:14, 1:12, and a 1:10 all set up to shoot very specific bullets out to certain distances.

Zerodrift,

You seem to know your 6mms My 6mm rem deer gun will have a 1in10 (standard shihlen)twist and allthough primarily for 90/100gr bullets I wanted to be able to shoot the 105gr speer flat base spitzer at not top velocity. Do you think a 10 twist can handle these at 2,900fps up to 300yards? I had wanted 9 twist but shihlen do 10 and say it's fine even for the 115gr barnes RN.

[This message has been edited by 1894 (edited 03-12-2002).]

1894 - You will be fine with a 1:10 and 105gr + bullets. I have not run the calculations, however, you would only begin to find a stability issue past 500-600 + yards. Shilen makes a great 6mm barrel. I have one Shilen and two Krieger barrels in 6mm.

I have shot 105gr Bergers out of my 1:12 with no real accuracy problems out to 200 yards. Not recommended but it did work.