Barrel twist

I'm a new B/P shooter and have been getting along alright. However I've been reading the forums and have realized there's a lot going on that I haven't really thought of.

I'm just wondering what is the relation to barrel twist and bullet caliber or length? I'm shooting a 490 patched ball out of a 1:48 twist barrel. And I know a conical bullet needs more of a twist for stabilization. I am wondering if there is some formula to relate bullet length to twist rate.

I'm just trying to understand some of the ballistics here.

 

[Trench]

Hang on to your seat, because the members of this forum are going to knock your socks off with information! Paul, whip out those formulas! :grin:

 

That's what I was hoping for :grin:

 

[chapmans]

The Greener formula:

Twist = 150 divided by length in inches times diameter.

Or Length of bullet = 150 times dia squared divided by the twist.

 

[Dphar]

I'm a new B/P shooter and have been getting along alright. However I've been reading the forums and have realized there's a lot going on that I haven't really thought of.

I'm just wondering what is the relation to barrel twist and bullet caliber or length? I'm shooting a 490 patched ball out of a 1:48 twist barrel. And I know a conical bullet needs more of a twist for stabilization. I am wondering if there is some formula to relate bullet length to twist rate.

I'm just trying to understand some of the ballistics here.

Short conical will shoot fairly well from 48" twists but are not very stable and if used for hunting sometimes will go off track on striking the animal.
You would do better with a 30" maybe 36" for conicals but too fast is not all that great either.
Search for Greenhill formula on Google. It will give a formula. But you need a bullet length and to understand that this formula is designed for artillery.

Dan

 

[mazo kid]

You are already shooting PRB out of your 48" twist barrel, and I'm assuming that is going OK. Are you trying to get a heavier bullet to shoot for hunting? I would suggest trying to find someone who will let you shoot a few of the Lee R.E.A.L. bullets in 50 caliber (in your rifle) so you can see for yourself. Some rifles shoot them quite well, others say they need a faster twist than 1 in 48".

 

[tg]

"? I'm shooting a 490 patched ball out of a 1:48 twist barrel."

If you are getting good accuracy with that combo ...don't fix it is it ain't broke, bullets are not needed for any game you can hunt with a .50, you will get a few pages of responses I suspect I would suggest staying with the traditional PRB if it does what you need it to do, I am certain you will get a lot of different ideas, trial and error will give you some answers, modern bullet shooters will give you some others, in time you will be able to sort it all out.

 

[paulvallandigham]

The general formula used for determining the LENGTH of the bullet( conical) for a given bore diameter, is known as the " Greenhill Formula".

Caliber Times the Caliber, times 150 divided by the length of the bullet will give you the optimum ROT for that bullet length. ( This formula works for guns that have muzzle velocities of 1500 fps, or less.)

If your gun is going to shoot a bullet( Lead) at a speed more than 1500, you should alter that formula by using a number, rounded up to the nearest 100, that is 1/10th of the velocity of the bullet. So, for instance, if your bullet will be traveling at 1800 fps, then use "180" as your multiplier.

To find the Length(L) of bullet to use with a given caliber and Rate of Twist( ROT)

multiply 150 times the caliber squared( caliber times caliber) and then divide that number by the rate of twist( 1:48 = "48" for the denominator.)

For a .50 caliber barrel with a ROT of 1:48, I get a bullet length of .78" (L= 150 X .50 X .50 DIVIDED BY 48= .78125")

There are other formula out there being used. The Greenhill formula dates to the 19th century, and most "experts" feel its probably the best to use with lead conicals. The modern rifle "experts" are not quite too eager to agree when it comes to high speed copper-jacketed bullets.

That being Said, the copy of the formula I have came from an issue of Rifle Magazine, and the tablebegins with .17 caliber bullets, and ends with the .458 diameter bullets. This table was obviously made to address the needs of modern cartridge rifles, and not traditional BP rifles.

IMHO, its kind of nice to learn that a formula that was worked out in the age of Black Powder is still applicable to modern cartridge rifles( regardless of what some experts now say). :grin: :rotf: :rotf: :wink: :hatsoff:

I hope this helps answer your question.

 

[Zonie]

The Greener formula:

Twist = 150 divided by length in inches times diameter.

Or Length of bullet = 150 times dia squared divided by the twist.

First let me thank you for your post.
Not to be overly critical, but it has a few little errors in it.

The name of the formula is "Greenhill Formula".

"Twist = 150 divided by length in inches times diameter" should read:
Twist = 150 divided by length in inches times the diameter squared.

Now, I know that Stumpkiller was too busy in Algebra studying Amy's strapless top so he missed it so I better point out for him that to square a number you multiply it times itself so 3 squared is just saying 3 X 3 = 9. :grin:

 

[jtmattison]

An elongated bullet flying without spinning will be unstable and inaccurate. The longer the bullet is in relation to its diameter, the more spin is required to stabilize it. How much spin? This is expressed in the Greenhill formula, a simplified version of which is:

150 x diameter squared divided by bullet length = required spin

Example for a .45 caliber bullet .60 inches long:
150 x .45 x .45 divided by .60 = 50.6 inches
So, for the example bullet, a spin rate of 1:50.6 or faster is required

The formula can also provide us with the maximum bullet length which can be stabilized by a given barrel twist.

The formula becomes:
150 x diameter squared divided by twist rate.

Example for a .50 caliber barrel of 1:60 twist:
150 x .50 x .50 divided by 60 = .63 inches
The barrel will stabilize a bullet .63 inches long, or shorter

Example for a .50 caliber barrel of 1:28 twist:
150 x .50 x .50 divided by 28 = 1.34 inches
The barrel will stabilize a bullet 1.34 inches long, or shorter

Example for a .54 caliber barrel of 1:48 twist:
150 x .54 x .54 divided by 48 = .91 inches
The barrel will stabilize a bullet .91 inches long, or shorter

Example for a .54 caliber barrel of 1:60 twist:
150 x .54 x .54 divided by 60 = .73 inches
The barrel will stabilize a bullet .73 inches long, or shorter

Example for a .54 caliber barrel of 1:32 twist:
150 x .54 x .54 divided by 32 = 1.37 inches
The barrel will stabilize a bullet 1.37 inches long, or shorter

HD

 

[cowpoke1955]

I'm with mazo. There is very little a .490 ball won't take down. Lee makes two weight of REALs in .50 caliber, 250 grain and 320 grain. I know some guys who shoot the Lee REAL and T/C Maxi-Balls out of their .50 cal. with 1 in 48" twist barrels. Each rifle is different so just try them. If it doesn't work out, you still have your round balls. Good luck :thumbsup: .

 

[user 13289]

For deer sized vitals at ranges appropriate for a 50 cal PRB--don't worry so much about the math. Just work up a load and be consistent with your loading parts and procedures.

IOW-HUNT and enjoy the woods.

Of course you want your rifle to be as accurate as possible, but now isn't the time to be splitting hairs over twist rate-provided you have hunting level accuracy already.

Come back after season and we'll work on those one-ragged-hole groups. :v

 

[longcruise]

IMHO, its kind of nice to learn that a formula that was worked out in the age of Black Powder is still applicable to modern cartridge rifles( regardless of what some experts now say).

You can quickly and easily prove that it does not apply to modern cartridges. Just measure a 200 grain '06 bullet, a 150 grain .270 bullet and a 140 grain 6.5mm (.264) bullet and then enter those lengths into the Greenhill formula and compare to the actual twist rates successfully used for decades in those calibers.

Here's a couple examples:

Sierra .224 60 gr HP, Green hill says it needs a 9.58 twist. Shoots sub moa all day long from a 12 twist.

Rem 150 grain 7mm (not even the favored longer 175 grain), Greenhill says it needs a 9.84 twist. Shoots great from a 10 twist 7-08.

Speer 150 grain .270, Greenhill says it needs a 9.71 twist. Drives tacks from a 10 twist. The same 10 twist shoots a longer 160 grain bullet accurately.

All of the above are lighter (shorter) bullets than will shoot from the barrells cited here. I just don't have the longer ones on hand to measure.

The fact that Rifle magazine printed a chart does not mean a thing.

As Dan said, Greenhill is an artillery formula. Not even applicable to BPC bullets and not very useful for ml bullets either.

If ya wanna know if it will shoot, ya gotta do the shooting.

 

[paulvallandigham]

Do you really think that anyone cares here about your ideas about modern rifle bullets, or artillery shells? You seem to want to take these posts off topic so you can prove what?

Its obvious that you are so much smarter than me, that I should just refer any questions to you in the future. No? :hmm: :surrender: :thumbsup:

 

[longcruise]

Do you really think that anyone cares here about your ideas about modern rifle bullets, or artillery shells? You seem to want to take these posts off topic so you can prove what?

That's what I thought when you brought up modern rifle bullets! :rotf: Just staying on your topic! :blah:

You proposed the Greenhill formual as the solution to bullet length. I just posted factual information that proves it ain't so. Still your topic! :blah:

Its obvious that you are so much smarter than me, that I should just refer any questions to you in the future. No?

Yes!!

 

Thanks for the reply's as it was almost exactly what I was looking for.

As for now I don't plan to start hunting as I haven't done it before and need to find some people that would be willing to take me out with them. Anyway I have no idea what I would do with a deer right now anyway. :wink:

I'm pretty happy with my PRB at the moment for the target shooting I do and see no reason to change. Rather, I just wanted to get a further understanding about how everything works together.

Maybe I'll start looking into some conical just for fun soon, and defiantly want to look at hunting at some point in time. This info may come in handy once I do. :thumbsup:

 

[paulvallandigham]

The Greenhill formula is based on Bullet Length, not weigh. Faster ROTs will stabilize heavier, and longer bullets, but they can still shoot lighter bullets well, too.

Hollow pointing a bullet shifts its center of gravity rearward, and that also affects the stability of a bullet.

As I previously noted, the Greenhill formula is based on the idea that 1500 fps. MV is going to be the optimum velocity for Cast Lead Bullets fired from a rifle. In the larger caliber guns of the day, it was much harder than it is now, to use black powder to reach velocities with conical bullets that exceed 1500 fps. I would like to think that we would no fault the authors of the Greenhill Formula because they didn't anticipate smokeless powder, and the higher velocities it permits us modern shooters to achieve with rifles.

If you adjust the formula for the velocity of Modern Jacketed Bullets, the formula gives better data for MJBs, and the guns that fire them.

For example, and only because you mention a cartridge with which I have some familiarity, one of the bullet companies made both a 100 grain, and an 85 grains HP bullet in .264" caliber. The length of both bullets were exactly the same. The outside shape of the jacket was EXACTLY the same.

The difference was in the hollow point, which obviously removed the additional weight.

In actual shooting, the two bullets hit the same POI at 100 yds. A friend found that out using all 5 of his 6.5 x 55 mm Swedish Mausers, by firing a 5-shot group with each bullet from each gun, but firing both the 85, and 100 grain bullets on the same target. Each group was under 1 inch. Both groups together were under 1 inch.

BTW, Length of a Nosler, 140 grain BTHP bullet is 1.318". I don't have either the 85 or 100 grain bullet to measure, but they will be shorter than this longer, and heavier bullet.

Using the straight Greenhill formula and a ROT of 1:10", the rifle will stabilize any bullet at least 1.04" long(L= 150 x .264 squared, divided by 10). if you adjust it to reflect the higher velocity that these bullets are fired from modern rifles, say, 2400 fps, YOu find that the 10 inch ROT will stabilize a bullet up to 1.67" long. ( L= 240 x .264 squared divided by 10), which is much longer than the 140 grain Nosler BTHP bullet. There is a 160 grain bullet that is used in Europe and Africa for truly big game( moose, and tigers), and I must assume its also longer in length than the Nosler bullet I have.

The Greenhill formula does not say you can't stabilize shorter( and presumably lighter bullets). It simply proposes that longer bullets( presumably heavier) for a given caliber, need a faster ROT to stabilize.

My .30-06, with its 1:12 ROT will shoot the very short 100 grain and 110 grain .30 caliber bullets, although I have to be careful about velocities with these small bullets, or they can keyhole. It will handle up to 200 grain bullets very nicely. Velocities range from 1300fps for my 100 grain plinkers, to 2700 fps. for my 150 grain hunting bullet, down to 2400 fps for those larger 200 grain hunting bullets. I have tried some 130 grain bullets at 3,000 fps, and they were accurate enough, but I didn't like the pressure signs I was getting with the load, so I backed it down, and then abandoned it, as I have other rifles to use to hunt game suitable for that bullet.

So, I think you misunderstand the Greenhill formula- as do other modern rifle shooters. It was crafted to figure out how to stabilize long projectiles, without concern that a faster ROT would also stabilize short bullets. If you make the adjustment for the velocity of the caliber and cartridge you are going to load, the formula is still quite useful. YOu can shoot the LEE Precision, R.E.A.L. bullet out of a fast twist barrel, but the longer Hornady conicals might not stabilize properly from a slow twist barrel. The Lee bullet, however was designed to be used in the slower twist barrels. Most of the heavy conicals require ROTs of 34 inches or less to stabilize well.

Rather than debunking the formula, your diatribe simply confirms it.

Try again. :thumbsup:

 

[Dphar]

Greenhill.
One must remember that it is meant for a given bullet shape and BP velocities.
In BPCRs it is generally best to tighten the twist Greenhill gives by 1-2". If it gives a 20" use an 18".
For the patched round ball hand wringing over twist is silly generally any round ball twist will work fine once the load is found for the barrel.
Bullets in MLs are another thing.
Just because a X twist will stabilize a X long bullet does not mean its actually USEFUL.
Why? A marginally stabilized bullet may shoot well but if used for hunting it may be less that optimal. Marginally stable bullets tend to veer off track when striking flesh. This has been known since at least the Crimean War.
If you feel you must shoot elongated projectiles at game then put sufficient twist on them. Something in the 30" range.
But REMEMBER that a RB of equal weight will likely be more effective on game within its range than the conical.
The conical is not some panacea and the "naked" conical/Minie never found favor with anyone but the worlds militaries. It could not even be used by the cavalry since it would unload itself if the carbine was slung.
ML hunting rifles used the patches RB or the cloth patched picket in the 19th century. It was not until the advent of modern HV jacketed bullets that bullet energy came to be viewed as a way to judge killing power.
Lead bullets at BP velocities are generally found deficient when judged by this method. But its the fault of the METHOD not the projectile. See the thread on RB ballistics in the General MLing forum.

Dan

 

[roundball]

From my personal experience with various T/C standard 1:48" and round ball 1:66" twist barrels:

The .50cal x 1:48" twist is an excellent twist for both the .490" PRB and conicals like the Hornady 385grn Great Plains HP (or FN) bullet.
While I've only used PRBs for years now, I have used the Hornady GP in both .45 & .50cals for deer hunting and in both cases, using an Oxyoke prelubed wad over the powder, accuracy was outstanding, and their reputation on deer is well established.

Many will spout an old wives tale that 1:48" twist won't shoot PRBs "accurately" but it is a false across the board statement for a few reasons:
1) because twist is caliber oriented...for example, 1:48" twist "IS" the round ball twist for the .40cal.
2) what is meant by "accurately"...a match competition shooter might want that extra 1/2" tighter group at 100 yards and will use a slower round ball twist barrel, but the average weekend shooter/hunter won't ever notice the difference in accuracy because there is simply not that big of a difference between a 1:48" vs. a 1:66"...it is so slight that it's normally overshadowed by all the other shot to shot variables of loading and executing a muzzleloader shot.

The flip side is that if you go to a slow twist round ball barrel, conicals rarely ever shoot accurately out of them.

 

[Mad Professor]

I'm a new B/P shooter and have been getting along alright. However I've been reading the forums and have realized there's a lot going on that I haven't really thought of.

I'm just wondering what is the relation to barrel twist and bullet caliber or length? I'm shooting a 490 patched ball out of a 1:48 twist barrel. And I know a conical bullet needs more of a twist for stabilization. I am wondering if there is some formula to relate bullet length to twist rate.

I'm just trying to understand some of the ballistics here.

Short conical will shoot fairly well from 48" twists but are not very stable and if used for hunting sometimes will go off track on striking the animal.
You would do better with a 30" maybe 36" for conicals but too fast is not all that great either.
Search for Greenhill formula on Google. It will give a formula. But you need a bullet length and to understand that this formula is designed for artillery.

Dan

I'll emphasize sometimes.

Before I knew better, I hunted deer with a 1-48 58cal with 525gr hornaday over a max charge. I've put that straight though a deer and then straight through a 4" maple and it was still going straight.........where it stopped????