Twist vs Projectiles

[rodwha]

My .50 cal Lyman's Deerstalker has a 1:48" twist with deeper grooves (~0.520") meant for RB's. But I also see that Lyman's made their Maxi specifically for rifles with this twist rate.

I was told that heavy pistol bullets in sabots wouldn't stabilize as well as a typical (250 grn .45) bullet at longer range.

So what is it about twist and projectiles? Is it the weight, speed, or length that makes the differences?

It's suggested by Lyman's then that their rifle with the 1:48" twist works well enough with both a light, short RB, as well as a heavy long conical. So could it also be that any twist could work well with say a very light, but not an intermediate, and again with a heavy also?

I've read this same sort of thing with powder charges. For instance a 60 and a 100 grn charge could be very accurate, though in between it falters.

 

[Doak]

Most of the literature I've read, all my life, always says heavier bullets need to spin faster to remain stable over longer ranges.

It's more accurate to state that longer bullets must spin faster to remain stable, etc.

The only way I know of to make a bullet, of a given caliber, heavier, is to make it longer, assuming the materials in the bullet, i.e. lead, copper, etc. remain the same.

If the bullet is made larger in diameter, to make it heavier, then it's not the same caliber anymore...:-D

Having a long axis of rotation encourages the bullet to destabilize & tumble if it's rpm's aren't high enuff.

The ballistic coefficient of a bullet goes up in value as that bullet gets longer, requiring said bullet to spin faster.

Longer bullets, in a given caliber, have more mass, and carry their energy & velocity further down range, and buck the wind better, assuming they're spinning fast enuff to remain stable.

Velocity effects the trajectory of a bullet, more than it's stability. Higher velocity means higher rpm, granted, but higher velocity can't compensate for a twist rate that's too slow for a longer bullet at any velocity.

Kindest Regards,
Doak

 

[rodwha]

Thanks Doak. It does seem to be determined by length.

"If the bullet is made larger in diameter, to make it heavier, then it's not the same caliber anymore..."

Unless a sabot is used, which is partly where I was going with this. I've considered 240 and 300 grn .44 bullets, 265 grn .444 bullets, 240-300 grn .45, and 300-400 grn .458 bullets in sabots, as well as full bore conicals. I lean towards Hornady bullets as I like their mix of quality and cost, but I'd also consider, and am, lead cast bullets.

 

[Rojelio]

I don't know how the greenhill formula applies to muzzleloaders, but, here it is.






Greenhill Formula for Rifling Twists

The Time-honored Greenhill formula was originally used for determining twist rates in the new rifled artillery in the 19th Century.. It is important to note that it is projectile *length*, not *weight* which is the determining factor when considering the best twist rate for your intended use. Weight can vary for a specific bullet length from use of different material and also is determined to some degree by projectile shape. Often long-range match shooters utilizing low drag or VLD projectiles will voice a preference for "lazy" twists; i.e. just enough to stabilize the bullet. The requirements of a Shooter who is utilizing both high velocity and Subsonic ammunition are quite different.

When utilizing subsonic ammunition there is another factor to take into consideration. The projectile not only has only (about) 1/3 of the forward velocity of standard ammunition in Rifle calibers, it also has 1/3 of the *rotational* velocity...think "RPM's". This makes for a less gyroscopically-stable projectile, so a faster rate of twist is indicated than by simply applying Greenhill alone. This wisdom was imparted to me in a conversation with the great barrel-maker Boots Obermeyer. It makes perfect sense, and doubly so coming from him, so we'll take it as Gospel.

Greenhill Formula For Rifling Twists

T*B=150*Sqrt (Density of Lead/Density of Bullet)

T=Twist

B= Bullet length

Both units have to be in 'Calibers', density portion is optional

**********

To determine the *approximate* twist rate for a given projectile (this varies a bit by shape):

Using the a 190 gr Sierra MatchKing bullet #2210M (.,30 cal) as an example. If we measure the bullet's length it is 1.375" long .

B=Bullet length in *Calibers* (Length/Diameter), hence 1.375/0.308= 4.46 *calibers* long

From formula we have T=150/4.46=33.6. This figure is now in *calibers*, so we convert the twist from calibers to inches. So T=33.6 x 0.308=10.34".

This is why most commercial rifles in .300 Win. Magnum come with a 1:10" twist barrel.

***********

One can go further and get involved in formulas for density, but for most conventional lead/gilding metal projectiles, the above will suffice. Note that this is for a normal High Velocity loading. Subsonics generally require a somewhat faster twist rate. If one plans to use both High Velocity and Subsonic loads in the same platform, a good compromise which works acceptably for both must be found. If the platform is a "dedicated" gun in which subsonic ammunition alone will be used, then a faster rate can be utilized in order to be able to accurately shoot longer (and thus heavier) projectiles subsonically and with good accuracy.

Example:

Twist rates for 7.62mm/.308:

High Velocity only: 1:11-1/8" to 1:12"
Both: 1:10"
Subsonic ONLY: 1:8"

Bear in mind that "dedicated" back at the shop might not be "dedicated" in the field. Most knowledgeable end users prefer to have the option of utilizing both loads so as to be able to address changing tactical requirements on the spot.




Engel Ballistic Research, Inc.

 

[Rojelio]

Another one:
A Calculator for Barrel Twist Rate


inputs outputs
bullet length in twist in
bullet diameter in
muzzle velocity fps errors
bullet SG
bullet SG values: 11.3 lead
8.9 copper
8.5 brass
7.8 steel



--------------------------------------------------------------------------------


Notes

The classic Greenhill equation is

T' = 150 / L'

where the twist and the bullet length are in calibers. Removing bullet diameter from twist and length gives the equation often found:

T = 150 * D^2 / L

The Greenhill equation includes no term for muzzle velocity, and several sources suggest replacing the 150 with 180 for muzzle velocities over 2800 fps. Increasing muzzle velocity increases bullet spin, and spin provides the stability. An article in the 11/2001 Single Shot Exchange cites an article by Les Bowman in the 1962 Gun Digest offering an equation which includes muzzle velocity (in fps):

T = 3.5 * V^0.5 * D^2 / L

At 2800 fps, this equation is equivalent to using 185 in the Greenhill equation, and at 1840 fps, this equation is the same as Greenhill's.

Ken Howell wrote about twist rate in the 07/1999 issue of Varmint Hunter magazine. He mentioned Greenhill's work began with cannons in 1879. Two quotes Howell took from the Textbook of Small Arms (published in 1929 in Britain) are notable. "In actual practice Greenhill's figure of 150 can be increased safely to 200 and still control the bullet." The classic equation is for solid, lead alloy bullets of specific gravity (SG) 10.9, and "when the density of the bullet is less than that of lead or the density of the resisting medium is greater than that of air, the spin should be increased as the square root of the ratio of the densities." As SG decreases, the gyroscopic inertia of the bullet decreases in proportion, and one needs to increase the spin to compensate.

Howell feels one can overstablize a bullet. Ideally, the bullet's axis will keep tangent to the flight path, but overstablized, the bullet will instead remain pointing in the direction of the barrel. He offered no way to quantify such overstabilization.

C.E. Harris, writing in the 08/1983 issue of the American Rifleman, noted Greenhill's formula was developed before spitzer boattail bullets and high velocity cartridges. He used a more modern analysis of gyroscopic stability, in which a factor of 1.4 is minimum and 1.7 is usually good. He found that the numbers given by Greenhill's original formula ranged from 1.5 to 2.0 for military type boattail bullets and were about 2.0 for bullets with either a flat base or short boattails.

The basic twist rate calculator above uses Bowman's equation modified with the SG correction quoted by Howell. However, Don Miller has shown this older equation to not be accurate over the full range of bullet shapes and muzzle velocities. I plan to use Miller's more accurate twist estimator in this calculator.

John Knight in England offers a free Win32 executable, WinGyro, which provides a much more sophisticated analysis than done in the calculator here. In comparing results from this calculator to those of WinGyro, it appears that Bowman's correction can be too optimistic regarding the effect that muzzle velocity has on stability. Using a velocity of 1840 fps here (reducing the calculations to that of Greenhill's equation) sometimes gives a better match to WinGyro's results. Another fine tool for estimating twist rates (and ballistic coefficients) is found on the JBM site.

Gain Twist

Gain twist is a recurring fad. The theory is that with little twist at bullet start, the bullet will engrave easier allowing one to use more powder. The twist then increases as the muzzle is approached in order to give the bullet sufficient stability for flight.

One problem with gain twist is that the bullet must be deformed some as the engraving angle (of the rifling upon the bullet surface) changes. As for a potential increase in velocity, in the 6/1979 American Rifleman, W.C. Davis notes that the rotational energy of a bullet is a tiny fraction of the translational energy; for the .30-06, it's about 0.35%. Further, Harris, in the 7/1977 American Rifleman, mentions twist has less effect on pressure than either barrel wear or the dimensions of the chamber, throat, and rifling, and that "for practical purposes, the effect of a reasonable change in rifling twist upon pressure can be ignored."

From this, I concluded twist will have little effect on the velocity potential of any cartridge, and for what little effect there is on chamber pressure, one can fully compensate by using a powder of slightly different speed.

--------------------------------------------------------------------------------

 

[rodwha]

I measured up by ROA bullets, which range in weight and diameter (.452-.460"). When I convert back to inches from caliber do I use .50 since this is in a rifle or do I figure for it the actual caliber of the bullet?

A 255 grn RNFP is 0.638" long. If I multiply it by it's diameter of .460" I get a twist of 1:49.8", but if I multiply by .502" I get 1:54.3".

 

[Doak]

It's for the diameter of the bullet, since it's what's actually going down range.

Even if ya shot it out of a 3'' cannon...the diameter, length, & stability of the bullet, in flight, is what matters.

And it would still need the same rate of twist, regardless of what diameter bore it came out of.

 

[Rattus58]

My .50 cal Lyman's Deerstalker has a 1:48" twist with deeper grooves (~0.520") meant for RB's. But I also see that Lyman's made their Maxi specifically for rifles with this twist rate.

I was told that heavy pistol bullets in sabots wouldn't stabilize as well as a typical (250 grn .45) bullet at longer range.

So what is it about twist and projectiles? Is it the weight, speed, or length that makes the differences?

It's suggested by Lyman's then that their rifle with the 1:48" twist works well enough with both a light, short RB, as well as a heavy long conical. So could it also be that any twist could work well with say a very light, but not an intermediate, and again with a heavy also?

I've read this same sort of thing with powder charges. For instance a 60 and a 100 grn charge could be very accurate, though in between it falters.

the caliber (diameter) versus the length of the bullet (weight) determines necessary twist in most cases. The longer the bullet the most spin is going to be required. Spin comes from two sources... velocity and twist rate. In slow long bullets, like the 500 grain .45's for example, you need a twist of 1-18 or thereabouts. Round balls, on the other hand need very little twist with the 1:1 sorta dimensions..

A 1-48 twist doesn't work with LONG bullets hardly ever, but does work with shorter heavy bullets... like in the 58's, 1-48 works with 500 grain conicals quite well.

A 1-48 twist technically (according to the Greenhill formula) is optimal for a bullet 5/8" long in a 50 caliber or a round ball to some degree... but it is not optimal nor will it do much to speed the bullet as well as a longer twist.

50 calibers shoot long bullets with 1-24 - about 1-30 twist rates... the slower the bullet goes, the tighter the twist seems to be required... thus the 1-28 and 1-30's working well with faster velocities.

Some very short (by caliber comparison) saboted bullets work well in 50's as well with these twists because of the velocity available.

 

[Rattus58]

I measured up by ROA bullets, which range in weight and diameter (.452-.460"). When I convert back to inches from caliber do I use .50 since this is in a rifle or do I figure for it the actual caliber of the bullet?

A 255 grn RNFP is 0.638" long. If I multiply it by it's diameter of .460" I get a twist of 1:49.8", but if I multiply by .502" I get 1:54.3".

A .638 length bullet with a .460 diameter requires approximately a 1-40 twist at muzzleloader velocities. You have to be careful with formulas designed for bullets well into the 2000 plus velocities... muzzleloaders for the most part are 1000 feet per second slower with many loads.

Aloha..

 

[Rattus58]

I don't know how the greenhill formula applies to muzzleloaders, but, here it is.






Greenhill Formula for Rifling Twists

The Time-honored Greenhill formula was originally used for determining twist rates in the new rifled artillery in the 19th Century.. It is important to note that it is projectile *length*, not *weight* which is the determining factor when considering the best twist rate for your intended use. Weight can vary for a specific bullet length from use of different material and also is determined to some degree by projectile shape. Often long-range match shooters utilizing low drag or VLD projectiles will voice a preference for "lazy" twists; i.e. just enough to stabilize the bullet. The requirements of a Shooter who is utilizing both high velocity and Subsonic ammunition are quite different.

When utilizing subsonic ammunition there is another factor to take into consideration. The projectile not only has only (about) 1/3 of the forward velocity of standard ammunition in Rifle calibers, it also has 1/3 of the *rotational* velocity...think "RPM's". This makes for a less gyroscopically-stable projectile, so a faster rate of twist is indicated than by simply applying Greenhill alone. This wisdom was imparted to me in a conversation with the great barrel-maker Boots Obermeyer. It makes perfect sense, and doubly so coming from him, so we'll take it as Gospel.

Greenhill Formula For Rifling Twists

T*B=150*Sqrt (Density of Lead/Density of Bullet)

T=Twist

B= Bullet length

Both units have to be in 'Calibers', density portion is optional

**********

To determine the *approximate* twist rate for a given projectile (this varies a bit by shape):

Using the a 190 gr Sierra MatchKing bullet #2210M (.,30 cal) as an example. If we measure the bullet's length it is 1.375" long .

B=Bullet length in *Calibers* (Length/Diameter), hence 1.375/0.308= 4.46 *calibers* long

From formula we have T=150/4.46=33.6. This figure is now in *calibers*, so we convert the twist from calibers to inches. So T=33.6 x 0.308=10.34".

This is why most commercial rifles in .300 Win. Magnum come with a 1:10" twist barrel.

***********

One can go further and get involved in formulas for density, but for most conventional lead/gilding metal projectiles, the above will suffice. Note that this is for a normal High Velocity loading. Subsonics generally require a somewhat faster twist rate. If one plans to use both High Velocity and Subsonic loads in the same platform, a good compromise which works acceptably for both must be found. If the platform is a "dedicated" gun in which subsonic ammunition alone will be used, then a faster rate can be utilized in order to be able to accurately shoot longer (and thus heavier) projectiles subsonically and with good accuracy.

Example:

Twist rates for 7.62mm/.308:

High Velocity only: 1:11-1/8" to 1:12"
Both: 1:10"
Subsonic ONLY: 1:8"

Bear in mind that "dedicated" back at the shop might not be "dedicated" in the field. Most knowledgeable end users prefer to have the option of utilizing both loads so as to be able to address changing tactical requirements on the spot.




Engel Ballistic Research, Inc.

Use 120 instead of 150 for lower velocity missiles... say 1800 or less or maybe 2000 fps even...

Aloha...

 

[rodwha]

I find it strange and intriguing that Lyman's has designed their Maxi at 370 grns specifically for rifles with a 1:48" twist.

As Lyman's gives a max load of 3F as 90 grns with lighter projectiles, I assume and use Hodgdon's 80 grn charge as my likely velocity with the various projectiles they show, which would be ~1750 fps (240 grn .44/250 grn .45).

What full bore conicals/bullets do you guys use in your .50 cal rifle with a 1:48" twist? Bullets in sabots?

I don't typically see projectile length given to help in choosing. I see the 255 grn bullets I have come close to being ideal so I'd have to kinda guesstimate from there, and would think close to 300 grns for a .50 cal projectile.

Has anyone found that a FN or HP bullet did better than an aerodynamic profile of the same weight/size? Would, say a typical 250 grn FN .45 do better than a Hornady 250 grn .45 cal SST?

 

[Rattus58]

I find it strange and intriguing that Lyman's has designed their Maxi at 370 grns specifically for rifles with a 1:48" twist.

As Lyman's gives a max load of 3F as 90 grns with lighter projectiles, I assume and use Hodgdon's 80 grn charge as my likely velocity with the various projectiles they show, which would be ~1750 fps (240 grn .44/250 grn .45).

What full bore conicals/bullets do you guys use in your .50 cal rifle with a 1:48" twist? Bullets in sabots?

I don't typically see projectile length given to help in choosing. I see the 255 grn bullets I have come close to being ideal so I'd have to kinda guesstimate from there, and would think close to 300 grns for a .50 cal projectile.

Has anyone found that a FN or HP bullet did better than an aerodynamic profile of the same weight/size? Would, say a typical 250 grn FN .45 do better than a Hornady 250 grn .45 cal SST?

If you have a 255 grain bullet to be almost ideal, why change? Round nose or flat nose... both do extremely well on game... spire point hollow points can also do well on game and can also get plugged up and sail right through without being expanded.

I'm shooting at way less than 100 yards... less than 50 actually for most of my harvests and Ballistic Coefficient doesn't matter. If you're shooting long ranges well over 100 yards, then a light bullet or one with an exceptional Coefficient or have an accurate range finder, and accurate tables as to where to dial up your scope... Accuracy is usually the shooter.

As for the 370 grain maxi-ball even with soft lead they haven't done well for me so I use others. They are accurate in some rifles... I've a 50 caliber Thompson new Englander that shoots these things better than any of my other guns... so if necessary I've always got a back up... but I prefer other bullets over the maxi-ball and Lee's new minie kills better for me...

Aloha...

 

[rodwha]

I really would like to get into casting as these 255 grn bullets I speak of are sold at a premium when cast for you (price went up, but was $40/100 + shipping). And this is a part of the reason for my interest in learning to do so. Specific conicals are hard to come by such as REAL's and FN heavier pistol conicals.

I was actually interested in a lighter bullet for my pistol, but I do like things that can serve multiple purposes. Before this I was more interested in his 240 grn version mold.

As I'm not sure how one can clean a side lock well enough out in the field to get plastic residue out of a barrel I'm a little hesitant on using them, and would kinda prefer to use a full bore projectile.

So I've been considering paper patched bullets, Hornady FPB's, REAL's, and Lyman's Maxi. Maybe I should consider the Hornady PA conicals too...

I've never used a range finder, nor do I have much interest in getting one. Maybe if I lived out on the plains. I figure it's easy to set my zero so that I'm within 4" high/low and hunt the point blank range. I am no marksman, but with a good rest I can do well enough.