Survival: Advanced Marksmanship, Understanding Twist Rates and ballistics.

More than a year has passed since I first posted my intro to Basic marksmanship. Since then I've had several folks ask me to do more, take it to the next level...okay...Today it’s time to kick it up a notch or three and talk about something that a great many long time shooters do not understand well and that is Rifling and twist rates…
why this is so damn important to understand is most people buy the wrong kinds of ammo for their rifles then wonder why they cant hit the side of a barn... it might be the ammo your using

The Golden rule in ballistics is
The heavier a bullet is… the faster it must spin to stabilize in flight and the higher the velocity, the slower twist is required. yeah this is gonna get comp-lu-maa-kate-ed

Think of a gyroscope or more specifically Newton’s third law of motion. The spin keeps the bullet level in flight. When the M16 first came out the Army tried to use a bullet too heavy for its twist rate… the effect was the bullet would “Yaw” or tilt nose up as it hit the target. (Making a long slice in a paper target rather than a neat round hole) This made for very poor accuracy but the hyper-velocity; round hitting at a slight angle did cause devastating wound channels.
[atsimg]http://files.abovetopsecret.com/images/member/3727ec7f76ef.jpg[/atsimg]
Rule number two: Twist rates are measured in ratios:
Since I used the M4/M16 and its many clones as an example let me continue with that line and say, this style of weapon typically come in one of two twist rates 1:9 or 1:12
What that means is a bullet will make a full rotation 360 degree turn in nine inches, in the case of a 1:9… or one full turn in twelve inch, in the case of a 1:12…

As a general rule: The higher the velocity, the slower twist is required.
The lower the velocity, the faster twist is required. but bullet weight has a factor too...

That means the 1:9 puts a faster spin on a bullet thus allowing the user to pick a heaver bullet… while the slower 1:12 twist favors the smaller lighter bullets…
If you use the heavier bullets in the 1:12 you soon start to experience the “Yaw effect I was talking about earlier… and if you use the lighter bullets in the faster twist barrels they tend to wobble in flight. Opening up what would otherwise be a tight group.

Now it's time for the first of my many charts...

Caliber ...Twist
.172 10" For all bullets

.22 RF 14"* Twist for pistol barrels
16" Standard twist for rifle barrels
17"* Special twist for rifle barrels

.224 CF 9" For bullets heavier than 63 gr.
12" For bullets up to 63 gr.
14" For bullets up to 55 gr.
15"* For bullets up to 55 gr. driven 4,100 fps or more
16"* For bullets up to 55 gr. driven 4,300 fps or more

6mm/.243 8" Special for VLD bullets
10" For bullets up to 120 gr.
12" For bullets up to 85 gr.
13"* For bullets up to 75 gr.
14"* For bullets up to 70 gr.
15"* Special for bullets up to 70 gr.

.257 9" For bullets heavier than 100 gr.
10" For bullets up to 105 gr.
12" For bullets up to 90 gr.
13"* For bullets up to 80 gr.
14"* For bullets up to 70 gr.

6.5mm/.264 8" For bullets heavier than 120 gr.
9" For bullets up to 120 gr.

.270 10" For all bullets

7mm/.284 9" For bullets heavier than 140 gr.
11" For bullets up to 140 gr.

.307 13"* Special size and twist

.308 8" For bullets heavier than 220 gr.
10" For bullets up to 220 gr.
12" For bullets up to 170 gr.
14"* For bullets up to 168 gr.
15"* For bullets up to 150 gr.

7.65mm/.311 10" For all bullets

.338 10" For all bullets

9mm/.355 14" For low-velocity wadcutters
16" For all other bullets

.38/.357 14" For low-velocity wadcutters
18" For all other bullets

.358 14" For all bullets

.375 12" For all bullets

10mm/.400 16" For all bullets

.411 14" For all bullets

.416 14" For all bullets

.44 20" For all bullets

.451 16" For all bullets

.458 14" For all bullets
*Stainless Steel only
Black Powder Barrels (1.250" x 30")
.32 14" .320" groove

.38 20" .379" groove

.40 20" .403" groove

.457 20" .457" groove

The math for understanding the relationships between twist rates will make your head hurt… those formulas I will provide at the end of this thread, but for now I will offer a more practical method of determining the proper bullet style and weight for your weapon.

Now no two weapons are the same, even within a specific model line. So whenever I take a new gun to the range I take with me a small sampling of ammo. A mix of hollow points, boat tails and round noses as well as mixing up bullet weights… After doing my initial sighting in I get down to business.

At 100 yards I shoot my first group, from a solid bench rest… three rounds, starting with the lightest bullets I have… I give my barrel ample time to cool down… Cool down between groups is essential in measuring your weapons true potential…

To speed cool down, I have a small piece of cord I keep in my cooler in a plastic Ziploc so as not to get it wet. That stiff cold cord, I feed down the end of the barrel so I’m not waiting so long between groups…

Then shoot my next three round groups… I continue like this until I have shot all the different types of rounds I brought with me… making detailed notes on every group… I guarantee as you shoot your groups you will quickly learn what weight and style of round your rifle prefers… once you figure out the general weight your rifle likes you can then switch your focus to the different style of bullets and fine tuning to get the most accuracy possible… this is how people like me consistently shoot dime sized 100 yard groups…

Are you ready for some of that mind numbing Math I eluded too?

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.

Confused yet???
I'll make it simple...
Click here for an easy to use Bullet calculator
Scroll down about midway and you'll find some handy buttons to select for caliber twist rates and velocities... the answers come in a handy pop-up box...

Even more handy calculators can be found here at Ballistician's Corner
there you will find

A comprehensive collection of interactive, online ballistics calculators! Exterior Ballistics, Recoil Calculator, Wound Channel Calculator, Stopping Power Calculators (mutiple), Round Ball Weight Calculator, Powder Calculators, and more! A one-stop resource for your technical ballistic data needs.

>> Permanent Wound Channel

Marshall's adaptation and refinement of a formula to quantitatively calculate the permanent wound channel potential of any load utilizing a bullet with a relatively flat frontal area. This calculator generates its results by utilizing the meplat diameter (in inches) of a projectile and the striking velocity of that bullet to give an outstandingly concise calculation of the true permanent wound channel left by the bullet in live targets.

>> Relative Penetration Calculator

Another Beartooth Exlusive found only here! Generates the relative penetration index for any given bullet. Calculates based on bullet weight and Meplat or fronal area of the bullet. A great tool for comparing the potential penetration of one bullet to another.

>> Thornily Relative Stopping Power

Peter Thorniley designed this formula years ago. While hunting extensively both in North America and Africa he did practical field testing on numerous kinds of game animals. Living on the family orchard, he perfected this formula by continued field testing while conducting extensive depredation work. It calculates via a relative stopping power quotient the amount of gun and load necessary to cleanly take a game animal under all conditions. A great tool for comparison of one load to another, as well as the potential of one cartridge to another.

>> Taylor Knock Out Power (KO)

John (Pondoro) Taylor Knock Out Power has been respected over the years as a great comparison of two cartridges, one to another.

>> Foot-Pounds Energy Calculator

Figure foot-pounds energy of a load.

>> Recoil Calculator

Ever wonder how much kick a gun has? How fast it comes back at you? This calculator figures the velocity in fps of how fast the gun comes back at you, as well as how many foot-pounds are traveling with it!

>> Ballistic Coefficient (BC) Calculator

Want to figure the BC's for a bullet? This calculator will do the long equation figuring for you! Maximize before use.

>> External Ballistics Calculator

Figure the ballistics on a bullet/load combination! Includes bullet drop, lead, and more. Maximize before use.

>> Loads per Pound of Powder

Calculate how many loads you're going to get out of that new pound of powder.

>> Sectional Density Calculator

Calculate the sectional density of your bullet. (Proportion of length to width)

>> Cost per Load/Box Calculator

Calculate how much it costs to load your own! Figure either per load or per box! Includes primers, powder, bullets, and more! Maximize for viewing.

>> Powder Burning Rate Chart

Lists 110 of today's available canister propellants. Helps to get an idea of how fast or slow a given powder really is. Also available in PDF format. Click Here for PDF. Maximize for viewing.

>> Round Ball Calculator

Calculate the weight of a round ball by entering it's diameter! You can choose from four common alloys- Wheel Weights, Linotype, Pure Lead, or 1-10!

To end here are yet a few more Formulas you should be aware of...

THE FORMULA FOR COMPUTING WIND DRIFT

First some values:

t = time of flight in seconds
s = wind speed in mph
a = angle of wind off trajectory path
v = muzzle velocity in feet per second
r = range in yards

Wind Drift (in inches) = 12 x (88 x s / 60) x sin (a) x (t - (3 x r)/v)



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THE FORMULA FOR THE CORRECT SIGHTING OF A FIREARM

First some values:

H is the height of the sight above the bore in inches
D is the drop in inches of the bullet at sight-in range
S is the sight in range in inches
R is the range in question in inches
I is the drop from the bore at the range in question in inches
T is the difference in inches from the line of sight


If the range in question is less than the sight in range
T = (D-I) - ((S-R) x ((D+H)/S))

If the range in question is more than the sight in range
T = (D-I) + ((R-S) x ((D+H)/S))



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Kinetic Energy = ((Bullet wgt/225218) x Velocity)²/2



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Energy Transfer = Kinetic Energy x Caliber



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Optimum Game Weight = Velocity³ x Bullet Weight² x 1.5012 x 10e-13



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Mach = Velocity/1127



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Miles per hour = (Velocity x 3600)/5280



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Bullet Drop (inches) = 193 x (time of flight)²
(This formula does not take into consideration the effect of wind drag.)

Since everyone likes pictures let me add a few now to give you yet more info to chew on

Seesh that's a lot of information!

I'm crunched for time, so I will have to do more than just skim it-aka-read it in depth later. I love topics that help me learn more about the firearms that I use. That you for this information!

you forgot to mention head spacing, barrel crowning and floating

reply to post by anumohi


don't make me come over there and slap ya...
yes your very right in that too plays a huge role in accuracy...
but for now my fingers need a break before I make that thread LOL

If I may ask a question off the main topic, it is about shooting.
Is there any special technique for adopting left-handed shooting?
An injury left me legally blind in my right eye(can see but with no definition).
I grew up with guns so I am familiar . I just cannot seem to get the accuracy I used to have.

Awesome post. Very informative. Have been using basic ballistics calculators for years to calculate all my variables for years. Nice to see some of the math behind it.

reply to post by DaddyBare


A quick question - two calibers for which there are good amount of both guns and ammo floating around here in the US are 30/30 and 30/06. I don't see them in the table - should they be assumed to fall under the .308 listing?

reply to post by Hillbilly123069


when you go shooting try wearing an eye patch over your right eye for a while...
I'm only guessing but you might still be trying to use your right eye to shoot...
or the imbalance is throwing you off... covering the right eye might help... might...

Also do you wear glasses??? I have a hell of a time shooting with my bifocals on... there were times when the line in my bifocals lined up with the cross hairs and I could see the point of aim change as I moved my eyes up and down...

I had to have a special set glasses (non-bifocals) made up for me, just to shoot with

Originally posted by Frogs
reply to post by DaddyBare

 

A quick question - two calibers for which there are good amount of both guns and ammo floating around here in the US are 30/30 and 30/06. I don't see them in the table - should they be assumed to fall under the .308 listing?

.30-30 Winchester/.30 Winchester Center Fire/7.62x51Rmm
The .30-06 Springfield cartridge (pronounced “thirty-aught-six”, "thirty-oh-six") or 7.62 x 63 mm

now look at the first set of numbers "7.62" or .308... so yes your assumption is correct and I might add 30 cal ammo tends to be varied and available out of all the many different calibers

So, you've decided to let the public in our little shooters secrets have you?
About friggin' time
This is absolutely the most important thread on basic marksmanship I've seen here aside from your earlier thread on sight pictures. Even for all the shooting I've done these are points which are easily forgotten but can make a world of difference in your groups.
For all the crazy math which eludes me it seems a simplification to say that the faster and heavier the bullet the more time it needs to stabilize inside the barrel, thus the longer twist ratio required. (I hope I said that right!)
Or if your barrel has a short ratio then you should shoot slower loads or lighter bullets if accuracy is your #1 objective.

A good breakdown on bullet types would make a fine addition to this thread. Then we can discuss head spacing, crowning, floated barrels and glass bedding actions for those who want to go the extra mile or 10.

Thanks for a great contribution to the survival threads Daddybare
You do us old timers proud.

reply to post by Asktheanimals

A good breakdown on bullet types would make a fine addition to this thread. Then we can discuss head spacing, crowning, floated barrels and glass bedding actions for those who want to go the extra mile or 10.

My god man do you know what you asking???
Take Barnes

SOLID
TRIPLE SHOCK
VLC BULLET
X-BULLET
XLC BULLET

or these ones from Sierra
BlitzKing
GameKing
MatchKing
Pro-Hunter
Sports Master
Tournament Master

then you got Speer
AFRICAN GRAND SLAM
BOAT TAIL
DEEP-SHOK
GOLD-DOT
GRAND SLAM
HOT-COR
IDAHO TERRITORY
JACKETED HANDGUN
LEAD HANDGUN
MHP
SPECIAL PURPOSE
TMJ
TNT
TROPHY BONDED BEAR CLAW

notice I never touched on those from Remington or Federal or Hornady or, or or or or...
Then everyone has their own fav's Mine being ones I make myself with a Corbin jacket press...
Better to let folks tell us what they want their rifles to do then we can assess and make recommendations...

the days when the only choices we had were hollow points round noses and boat tails is thankfully... long gone

Yep and yep.

All roads lead to the idea that my M14 is better then the M4/16.
Oh yeah!!!!!!


Great work.

reply to post by macman


Okay lets compare the two...
by far the M14 is more rugged and reliable due to it blow-back design
however the M4/16 is lighter and as a modern weapons platform is a tad more versatile...

but that is talking about the guns themselves...

when you compare the cartridges yes the .308 is far superior performance wise..
but for a Never fired a weapon, first time shooter...the 5.56 is easier to learn to use with minimum amount of training...

the 5.56 lighter thus allowing men to carry more rounds... but the .308 has greater range and much higher knockdown...

we could go back and forth like this all day... both of these guns were born back in the 1950's and as far as I'm concerned better weapons and cartridges have come along since...

one of my favorites to come along in years is the Panther™ LR-243 A .243 win (6mm) it has a lot more kinetic energy than the 5.56 yet if light and able to produce double taps thanks to it's light recoil...

I remember your thread on the army field testing a new carbine... who knows, maybe the next gen will be one of the 6mm's???

reply to post by DaddyBare


I agree that the AR/M variant is a good 'first timer' platform.
I also agree that my beloved M14 is a more robust and stout platform.
I have used both. And I love me some M14.

I have also shot the .243. I am more along the lines of it being used as a hunting round. It is probably just my jaded way of looking at it.
I do like the idea of the 6.8 rounds that are becoming more popular.

reply to post by anumohi


I think DBare was just talking purely about ballistics.. Yes you're right that ballistics are not the only factors involved in accuracy but this thread is about ballistics man.

To DBare:

You're definitely right when talking about bullet weights and even brands of ammo (if you do not hand load that is) making your accuracy vary.

For example my Winchester model 70 pre 63, 30-06 loves Winchester silver tip 165 gr rounds.. You can stack groups at 150 with those (with federal 165's being it's second favorite factory loads).. However if you fire any 180 gr bullet you are lucky if you even hit the target. For my hand loads I usually go slightly hot with a 165 ballistic tip for max accuracy... These rounds have a superb Ballistic Coefficient and are my long range (750+ yard) rounds.

Originally posted by DaMod
reply to post by anumohi

 

I think DBare was just talking purely about ballistics.. Yes you're right that ballistics are not the only factors involved in accuracy but this thread is about ballistics man.

without proper head spacing of the round and the bolt you can forget all about accuracy and all the ballistic jargon you want

Just because there weren't enough fingers in this KoolAid...

The rifling ratio probably should be kept away from caliber and spoke of more frequently with size of the round. Example: 30-06 can be loaded anywhere from 100g to 220g. That is nearing 6mm to 338 grain-wise. Now I know they will not act the same as a 6mm or 338 but the rifling ratio, for optimal performance, probably should change coming out of the same rifle setup. Obviously, I shoot one rifle in 30-06 and I do not change barrels with the load. Call me cheap.

One poster mention the rate for M4/M16. Well is it in .223 or 50 Beowulf? Different rates.

reply to post by DaddyBare


Wow, thank you for this incredibly in depth and informative post! I knew that twist had a lot to do with stabalization and accuracy but now the details are crystal clear.

Quick question if you don't mind answering:

What is your opinion on the WWII M1 Garand? What twist ratio does it have (i'm assuming it's closer to 1:9) and what ammunition would you suggest?

Excellent information. As a kid I remember being amazed finding 30.30 win western. After a miserable time with remington and a single pellet shotgun. My nextdoor neighbor had the same ted willaims rifle as me and remington was flawless for him.

Now that I think about it there must have been a huge sale at Sears that year, cause lots of kids had the same lever gun.

Great thread! But are we talking survival,make do with what you have, or Camp Perry. Which by the way is under full swing. Come on out between now and August 17. The last 3 big matches are the Springfield, High Power and Long Range.

Good Times!!

Keep these good threads rolling.