Bullet grain v. Barrel twist

[RASteele]

What is the way one can tell what bullet size is best for their barrel twist?

[Rocky Raab]

There are several formulas out there, the most commonly known being the Greenhill Formula.

In practice, they work acceptably, but aren't completely accurate because they don't take bullet shape into account.

However, twist rate is determined not by bullet weight, but by its length. The longer the bullet, the faster it has to be spun to stabilize it. The shorter (or blunter) the bullet, the slower the twist rate needed.

What are you trying to determine? I can plug in some numbers and calculate a workable twist rate. I need caliber, velocity and bullet length.

[Evan03]

85gr 6mm at 3200

sorry to steal the thread curious what rocky would come up for twist rate for that weight bullet at that speed.

thanks

Evan

ps
im guestimateing 10 twist is about right for that bullet maybe 9

[Rocky Raab]

Evan...NOT weight. LENGTH.

[Catfish]

Burger list the recomanded twist rate for all of their bullets on their web page and you can use it for a good rule of thumb. As Rocky said though it is lenth and not weight that rules which doesn`t mean much with jacketed lead core bullets in the same cal and of close to the same shape. There will be some varation but not enough to make a difference in field shooting. Where you came get into big time trouble is when you move from jacketed lead core bullets to something else like the Barns X bullets which are solid copper and are much longer for the same weight lead core bullet. The longer the faster the twist needs to be. Velosity also plays a part since it`s rpm`s we`re looking for the higher the velosity the slower the twist rate of the barrel need be, but not by much.

[RASteele]

I just sent Hornady an email asking them to give me the lengths of their .308 IB bullets, so I probably won't hear anything from them until Tuesday.

The twist, however, is 1 in 9.5".

I was just wanting to get a little extra "humph" from my hunting ammo and I'd heard from someone that, unfortunately, "size does matter". At least when it comes to bullets...

[Catfish]

RA,
You will be looking at a 60 gn. bullet for best accuracy, or at least that`s my guess as most rifles designed for 60 gn. bullets use a 1 in 9 twist. You could get good accuracy with 55 gn. bullets also, but I don`t think you will be able to stablize the 69 gn. bullets.

[Martini25-308]

Are we talking about the total length of the bullet or the amount of the bullet in contact with the riflings??

[Rocky Raab]

Total length. The shape of the bullet (including the amount of bore-riding surface) does have an effect, but the Greehnill formulas doen't take it into account.

Without getting too technical, it's actually the distance between the center of mass and the center of aerodynamic pressure that controls stability - or the lack of it. The longer and "pointier" the bullet, the farther apart those two centers are, and the faster you have to spin it to keep it stable.

[RASteele]

Catfish was saying to use a 55 to 69gr bullet,... Are you talking .223??? Unfortunately, I'm needing it for a .308 diameter, specifically a .30-06.

Hornady is taking their sweet time getting back to me on this, so I'll email them again in the morning. Hopefully I'll hear something soon.

[Cal Sibley]

I think a good example might be my Remington Classic in 6.5x55. It has a 1 in 9" barrel twist and shoot the Sierra 160gr. Semi-Pointed bullet very accurately. Yet it's a real bummer with the Sierra 140 and 142gr, Spitzers. The lighter bullets in this instance are much longer than the heavier less pointed ones. Yet the 140gr bullets do much better in my older M96 Swedish mausers which have a 1 in 7 1/2" twist so weight isn't everything in determining what will shoot well. Bullet configuration plays a part also. Best wishes.

Cal - Montreal

[Catfish]

Sorry about that, thought you were takling abt. .22 cal. In .30 cal. 150 gn. would be the litest bullet I would try and the 168 to 180 range will problily shoot best.

[RASteele]

Thanks, Catfish! That's what I wanted to know. Now if someone could just give me the formula so I can apply it to other calibers, I'd be in hog heaven...

[Rocky Raab]

If you insist...

The formula is T=150(d/r) for velocities from about 1500 to 2800 fps. Substitute 180 for the 150 value for velocities exceeding 2800 fps. "T" is the twist rate. "d" is the bullet diameter. "r" is the bullet length to diameter ratio (length of bullet divided by diameter).

Or, just go here...

http://www.z-hat.com/twistrate.htm

Please note (AGAIN) that bullet weight has nothing to do with it.

[RASteele]

Outstanding!!! Thank you so much! That is exactly what I was looking for!