Hello:

Sorry to beat what may already be a dead horse but here goes:

I've seen conflicting comments on twist rate. Could someone please explain to me the relationship between twist rate and bullet length? I am building a 375 H & H and would like to shoot a 300 grain bullet as a general application bullet. However, I would also like to be able to shoot 350s or 380s for big game when the time comes for a trip overseas.

So, how does this work and what sort of a twist rate should I consider using?

Thanks!

Heavier longer bullets need a faster twice to stabilize. It is far more critical in the smaller calibers as increasing the weight of the bullet greatly increases the length. Once you get to the larger calibers it is normally a one twist (per caliber) fits all. Shilen calls the 1 in 14 for all bullets.PacNor calls a 1 in 14 or a 1 in 12 with the 5 grove barrel. I have no other experience.

I do question what advantage a slower moving 350 will give you over a faster good quality 300gr. Then again I have never tried to stop a fast approaching train with a rifle.

1 in 12" twist handles the job in the .375 H&H up to 350 grains but you might want a 1 in 10" for longer bullets.

Check your barrel maker to see if a 1 in 10" is offered.

Personally, if one thought he needed a 350 or larger bullet then he might wish he had a .416 instead....but I sure understand the desire for bullet weight and the old H&H has the "nuts" to throw it as well.

so, to be clear, a 1:10 is faster thus stabilizing a longer bullet? 2 issues ago Kevin Robertsen wrote in sports Afield that a 1:14 twist was appropriate for the 375 with 300 grain bullets at factory velocities according to the Greenfield formula. Hence my confusion.

Good question 3170.

1) It's not bullet length. It's bullet weight that is critical for this application. Bullets don't have to be longer to be heavier based on core material. However, in general, when bullets of a caliber get heavier they get longer because that's the only place the weight can go. They can't get fatter.

2) I have a 1:14 .375 H&H custom barrel from Kreiger. I wanted a 14" to help reduce felt recoil by reducing "torque" of faster twist. It stabilizes 300 grainers. I don't shoot any .375's over 300 grains. If I want more than 300 grains, I go up in caliber ie .416.

3)In any caliber a faster twist stabilizes a heavier for caliber bullet.

If you still have questions make another post. It's important you understand this subject.

quote:

Originally posted by Buliwyf:
Good question 3170.

1) It's not bullet length. It's bullet weight that is critical for this application. Bullets don't have to be longer to be heavier based on core material. However, in general, when bullets of a caliber get heavier they get longer because that's the only place the weight can go. They can't get fatter.

2) I have a 1:14 .375 H&H custom barrel from Kreiger. I wanted a 14" to help reduce felt recoil by reducing "torque" of faster twist. It stabilizes 300 grainers. I don't shoot any .375's over 300 grains. If I want more than 300 grains, I go up in caliber ie .416.

3)In any caliber a faster twist stabilizes a heavier for caliber bullet.

If you still have questions make another post. It's important you understand this subject.

Wrong Buliwyf,

It is LENGTH not weight that determines what twist is needed. Bullets that are long for their weight like some of the VLD types need a faster twist. I have a 1 in 14 twist .308 barrel that I can shoot stubby 170 flat points in or 180 cast but it will not stabilize a 168 HPBT. Why, because the heavy bullets are stubby while the HPBT is longer.

As forum members' posts have revealed there appears to be a discrepancy among opinions. Is this simply a mathematical/physics problem that can be demonstrated? I am going to be telling the gunsmith soon what barrel and twist to use so I'd like to be correct (and frankly, to make sure he is correct!).

with further regard to the Robertsen piece he said the 375 H & H at factory velocity in 300 grains is often "jawing" at 20 paces and after that settles down to the point where it becomes overpenetrative. He explains that he downloads the cartridge and the problem goes away.

Does someone have the issue at hand so they can qote directly rather than me paraphrase from memory?

Bullshit 721. Bullet weight. length is a non-issue.

quote:

Originally posted by Buliwyf:
Bullshit 721. Bullet weight. length is a non-issue.

Uh, ok.

Read this. I only spent a minute trying to find something for you to read.

http://www.outdoorlife.com/outdoor/shooting/article/0,19912,540468,00.html

The 6mm is a good caliber to demonstrate that length not weight determines twist. The 105 round nose will work very well in a 1 in 10 twist barrel but the LONGER Hornady A max requires a 1 in 9.

When I consider twist, bullet length is the only issue (other than the expected velocity) I consider.

Here you go Bullwif,

http://www.stevespages.com/page8e.htm

So easy, you should be able to understand it.

quote:

When I consider twist, bullet length is the only issue (other than the expected velocity) I consider.

As it should be. For most bullets of "normal" construction.

Buliwyf,
You might want to sit back and pay attention to Rem721.

From the link for velocities under 2800: twist rate = (150 * bullet diameter squared) / bullet length

For velocities over 2800: twist rate = (180 * bullet diameter squared) / bullet length

Now you might notice that there is no mention of bullet weight anywhere. There may be a difference of opinion on the issue but that does not alter the facts.

A copper bullet will have a much longer length than a lead bullet of the same weight and the copper bullet will need a much faster twist (smaller number) to stabilize it.

Thinking about it, length is important, and bullet weight can't be totally denied either. The twist rate is probably best determined as a function of Center of Gravity and Center of pressure, much like rocketry. The length will effect both the center of pressure as well as the center of gravity. Weight will only affect the C.O.G. I would also venture that bullet construction would come into play. Most rifle barrel makers reccomend their twist rates on bullet weights, which is too generic. My advice is to look to the bullet mfgs you wish to load and ask them for a reccomended twist.

John

Customstox & rem721,

Let me make this easy for you both. Call a barrel maker and discuss twist options. They will ask what bullet weight for caliber do you plan to shoot. They will not ask you how long the bullet is! If you buy a box of bullets it will be sold to you on bullet weight printed on the box, not bullet length! The rifling twist sees weight. Bullet length is all over the place, from a light weight Barnes bullet being longer than a heavier Woodleigh etc etc. It is not possible to standarize twist on length! What do you measure for bullet length ogive or meplat?? Bullet weight determines rifle twist!

Bullet length as a function of bullet weight is used for cutting throat depth on custom barrels. If you buy a 1:7" twist .223 it will have a longer throat than a 1:12" twist. Why? because with the 7" you are shooting 75-gr and up weight bullets and want to reach the lands not shove the bullet down into the case.

Bullet weight determines rifle twist!

Twist is often erroneously explained as being determined by bullet weight. Why, well, as you astutely noted, boxes of bullets are labeleed by weight, not length. The general public can identify by weight so manufacturers dumb down their discussions to terms most folks can understand. And, often enough these crude generalizations work. However, it is length, not weight, that determines what twist is needed. Check this out:

http://sst.benchrest.com/shilentwist.html

You'll note that the 6mm section clearly supports what I said yesterday. Look at the 8" and 10" twist section. You'll note that a 10" twist is recommended for bullets up to 120 grains, EXCEPT for VLD's which they relegate to use with bullets under 100 grains. Why? Because VLD's are LONG. I jokingly refer to VLD's as VERY LONG DESIGN.

Have you ever heard of the Greenhill formula? Look it up. Better yet, here:

George Greenhill, a mathematician at Emmanuel College, Cambridge, UK, developed a rule of thumb for use in calculating twist rates for a given lead-core bullet. The formula, named the Greenhill Formula in his honour, is:

where:

C = 150 (use 180 for muzzle velocities higher than 2,800 f/s)
D = bullet's diameter in inches
L = bullet's length in inches
SG = bullet's specific gravity (10.9 for lead-core bullets, which cancels out the second half of the equation)

Buliwyf,
Tell your smith you are using a copper bullet and see what reaction you get. The longer copper bullet weighs the same as your lead bullet but it will have a much greater twist. The Greenhill forumlae was based on metals having a density of 3.92 and above and was a simplication that served well given the parameters. A copper bullet does not fit the parameters but does need a much sharper twist.

You can use the Greenhill forumla for a give twist to determine the LENGTH OF THE BULLET that it will stabilize and there is not one ting in the formula for weight.

And just look at the formulae. It has NOTHING TO DO WITH WEIGHT.

The following link will let your determine the proper twist for a particular bullet. It will ask you to input the required information.

Greenhill formula solution for twist

Up front, I will say that I am not trying to do anything more than expand my knowledge, not take sides or twist panties.

How does bearing surface relate to twist? Greenhill was dealing with specific gravity of a bullet, which is a weight and weight distribution function, as well as the specific length. This weight function was used as the Greenhill constant, which is the 150 or 180 in the formulae. Definately NOT the same as the weight of the bullet, but the reason for the different constants that are used.

So my basic question is, since Greenhill uses a specific gravity, which does not take into account air temperature or air density, does a longer bullet with the exact same BEARING SURFACE length as a shorter bullet require the same twist rate?

How does bullet SHAPE factor into the formulae, or twist rate? Does a VLD bullet shoot more accurately over longer distances than a round nose because of bullet shape (aerodynamics) or because of stability? Again, I know over short distances bullet shape is almost irrelavent, but what about longer ranges?

Again, I am just trying to increase knowledge.

Since I started this thread Iwould like to get the discussion back on the track of original intention; I am searching for a barrel twist for a pair of matched rifles. We would prefer to shoot a 300 grain load for general hunting applications with range to 300 yards. As Africa figures into our plans I would also like to be able to shoot either 350 or 380 grain bullets for DG application. I would like to shoot the 300s at about 2450-2500fps and likely the 350s and 380s at 2200 fps. I am after a twist rate in waht will be 23" barrels to accomodate these needs if possible. For what it's worth I am not particularly interested in anything other than RN projectiles. I hope this helps.

Thanks again for all the interesting input!

If you were my customer I would recommend 1:12 twist. That's based on experience, the fact that Shilen only offers that bore in a 12 twist and based on a WAH (Wild Assed Hunch)!

The direct correct answer to your question is 1:12" twist.