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Old 11-27-2009, 10:54 AM
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Ballistic Coefficient Basics


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A ballistic coefficient is a number that tells how well a bullet slices through air. The number itself compares the effect of drag on a bullet in comparison to the effect of drag on a thoroughly studied standard projectile. This saves having to work out drag effects in detail for individual bullets or having to program the drag function of every bullet into a ballistics calculator. So, it is really a ballistic shortcut. Instead of measuring exact bullet behavior at all ranges and velocities, you simply fire a few to find the BC, then use that number to multiply or divide the standard projectile's behavior as needed to get your bullet's flight characteristics.

Drag determines how fast air resistance slows a projectile down. By using the BC to scale drag loses to your bullet, time of flight may be determined and, in turn, the amount of drop in bullet trajectory and the effect of wind may be calculated. The effect of drag on a projectile vary with velocity, but the idea is that because that has all been measured in detail for the standard projectile, the ballistic coefficient scaling of that recorded data will produce the effect of drag on your projectile at the same velocities.

Ballistic coefficients are based on standard projectile's that, by convention, are 1" in diameter and weigh one pound. It therefore has a sectional density of 1. As the standard it is declared to have a ballistic coefficient of 1. Projectiles that are slowed faster by drag than the standard projectile is, will have BC's lower than one. Those that are not slowed as quickly will have BC's greater than 1 (artillery shells, for example). All bullets that are the same exact shape will have ballistic coefficients with respect to that standard projectile that are simply equal to their sectional densities. Other, more or less aerodynamic shapes, will have a form factor to correct them for the fact they fly farther or shorter than sectional density alone would indicate in comparison to a standard projectile.

The effects of drag on those different shapes may be scaled to match the standard projectile pretty closely over any narrow range of velocities, but drag doesn't tend to change at the same rate with change in velocity for different shapes. As a result, if your projectile's shape does not match that of the standard projectile, the form factor will shift with velocity, changing the ballistic coefficient needed to scale the behavior of the standard projectile to fit. This is why Sierra and others publish multiple BC's for match bullets which change at different velocity limits.

For example, to find how far your particular bullet travels as it drops from velocity A to velocity B, just multiply your bullet's ballistic coefficient by the distance the tables show the standard projectile travels as it drops from velocity A to velocity B. That is what is meant by scaling your projectile's performance to the standard projectile's performance. (Hatcher's Notebook has the tables for the G1 standard projectile SAAMI adopted and for which most ballistic coefficients are commonly given.)

The calculation: Suppose you have a bullet with a ballistic coefficient of .462 (just to pull a number out of the air). You fire it with a muzzle velocity of 3100 feet per second. You want to know how far it will travel before its velocity drops to 3000 fps in standard sea level atmospheric conditions?* So, you look up those velocities in the standard projectile's tables, and find that it drops from 3100 fps to 3000 fps over a range of 100 yards in those conditions. You take that 100 yard figure and multiply it by your ballistic coefficient. The result is 46.2 yards. That's how far your bullet will travel in dropping from 3100 fps to 3000 fps. Obviously, the higher your bullet's ballistic coefficient, the farther it travels in losing that velocity.

Conversely, suppose you want to know how much velocity your bullet will lose in traveling a certain distance? You divide the ballistic coefficient into one, then multiply the distance you want to know about by the result. Use the tables to look up how much velocity the standard projectile loses over that resulting new distance, and you will know what your bullet loses? For the bullet in the first example, suppose you still have a muzzle velocity of 3100 feet per second. This time you want to know how much velocity it will lose going 100 yards? You divide the BC into one. 1/.462=2.16. Multiply 100 yards by 2.16 and you have 216 yards. Go to the G1 projectile tables. Start at 3100 fps and see how much velocity the standard projectile lost 216 yards later? The standard projectile, starting at 3100 fps drops to 2658 fps over the succeeding 216 yards. So, at 100 yards your bullet will be going 2658 fps.

You can use the ballistic coefficient not only to figure out how far a bullet will travel in dropping from one particular velocity to another, but also to figure out how much velocity your bullet will lose over a given range, or to figure out how long it will take to get to the target? That travel time is how much time gravity has to pull the bullet down off a straight line from the barrel, so it lets you calculate bullet drop. It also tells you how much more time it takes for the bullet to get to the target than it would do if the muzzle velocity stayed constant (as it would in a vacuum). That extra travel time due to atmospheric drag turns out to be proportional to the effect of a side wind on a bullet. It lets you calculate wind drift.

All external ballistics programs have the performance of the standard projectile under standard metro conditions built into them in tables and use your bullet's given ballistic coefficient(s) to calculate its trajectory in comparison to the standard projectile's. It adjusts the drag function for non-standard conditions of temperature, pressure, and R.H., all of which change the density of air.

The system of standard projectiles and ballistic coefficients is a shortcut that dates back to the second half of the 19th century. Artillery required a means of calculating where a shell would fall, but the mathematics for calculating projectile aerodynamics didn't exist then, and would have been too complicated to solve in the field without computers anyway. They were faced with having to spend years making thousands of measurements of each projectile, which would often be obsolete by the time they were completed. So they came up with this idea of using their crude (by modern standards) electromechanical ballistic chronographs to make thousands of measurements of a standard projectile, and determine its velocity loss ranges, fps by fps, over a very wide range of velocities, then using the ballistic coefficient to scale their other projectiles to its results. Tables in Hatcher's Notebook have them from 3600 fps to 100 fps. This Since it is easy to calculate a trajectory in a vacuum, this velocity loss information may be applied to adjust the vacuum trajectory, incrementally.

Today there are analytical methods that come from more comprehensive understanding of bullet aerodynamics, and computers can handle the volume of calculations needed to solve them in a reasonable time (once you've determined an individual projectile's drag function). But the work needed to make that determination for each bullet is more than the manufacturers of small arms bullets want to undertake, and it is more complexity than most users can make use of, so the old method persists for its relative simplicity.

The main problem with the old method is, as I mentioned earlier, that the real drag function of a bullet changes with its shape. For that reason the ballistic coefficient as a means of adjusting a bullet's trajectory really only works perfectly when your bullet has the exact same shape as the standard projectile the ballistic coefficient is referenced to. When the standard projectile is a big, heavy, flat base, small ogive radius, 19th century artillery shell, the match is seldom right for modern bullets. Nonetheless, it is the standard SAAMI adopted, called the G1 ballistic coefficient for French naval artillery's Gavre Commission which conducted a lot of the 19th century test firings. As a result, though you can find a matching ballistic coefficient over a narrow range of velocities for that old shape (this is just a curve fitting activity), it will tend to change over wider velocity ranges as the G1 drag function and your bullet's actual drag function diverge. As a result, you see manufacturers give tables of ballistic coefficients for different velocity ranges or, as Berger now does, they give a second ballistic coefficient referenced to a more similarly shaped standard projectiles, like the G7 standard projectile. An example is the Sierra 168 grain MatchKing's ballistic coefficients for the G1 projectile from the Sierrabullets.com site, copied, below.



That information will let most trajectory programs get reasonably close to the performance of that bullet, but it isn't perfect. The Army Ballistic Research Laboratory, came up with a compromise alternative to determining the drag function of each individual small arms projectile. They fired a series of different shaped standard projectiles so one may select the standard a particular bullet's shape is closest to. Using the ballistic coefficient determined for that closer shape lets you make more accurate trajectory calculations than the G1 ballistic coefficients does, even with velocity range adjustments. Those and others have been worked out over the years. Some are listed below.

G1 or G1.1 in last version, (Flat base, 2 caliber ogive, SAAMI adopted and the default published type)
G2 (Aberdeen J projectile)
G5 or G5.1 (short 7.5 boattail, 6.19 caliber tangent ogive)
G6 or G6.1 (flat base spire point, 6.09 caliber secant ogive)
G7 or G7.1 ((VLD type long 7.5 boat-tail, 10 calibers tangent ogive)
G8 (flat base, 10 caliber secant ogive)
GI (Ingalls tables projectile)
GL (blunt lead nose, like a soft point tubular magazine bullet)
GS (Spherical, measured with 9/16" projectiles)
RA or RA4 (.22 Rimfire standard projectile)

Berger publishes both G1 and G7 ballistic coefficients. The numbers are not comparable because the shapes of their standard projectiles are not comparable, though, in general, G7's will be smaller numbers for a bullet than its G1 numbers because the G7 standard projectile looses speed more slowly than the G1 standard projectile. You can use the G7 BC in the trajectory tables of the free online JBM calculators. Also, RSI's Ballistic Lab software and QuickTARGET Unlimited software will work with those alternative BC types.


Tech Corner

If you want to figure out the G7 or any other number for a bullet like the Sierra bullet in the table above, you can get pretty close using the free JBM online calculators. Look at the middle two BC numbers in the table. They have both upper and lower velocity limits. Pick one of those two ranges. Use its limits with JBM's trajectory calculator for the G1 BC. Note the distance traveled starting at the first velocity and ending at the next. Now plug those same two velocity limit numbers and the distance you noted into JBM's BC calculator and pick the standard you want the new BC for (G5. G7, etc)? The returned number should be close and in trajectory programs that have the other BC types available to use, should give you better trajectory predictions outside that velocity range than the G1 BC does.

For example: Using the first BC limits of 2600 fps and 2100 fps and the G1 BC given as .447, I run the JBM trajectory table for G1 in one yard increments to 300 yards (enough to drop to 2100 fps). I start with a muzzle velocity of 2600 fps, setting the chronograph distance to zero. The resulting table starts at 2600 fps and scrolling down I find 2101 fps at 262 yards, and 2099.2 fps at 263 yards. I extrapolate to get 262.6 yards as the point at which velocity was 2100 fps.

Next I go to the velocity-based ballistic coefficient calculator. I plug in a start velocity of 2600 fps and an end velocity of 2100 fps. I put 262.6 yards (don't forget to select yards; default is inches) into the distance. I run it once with the G1 number selected to be sure it returns the same 0.447 BC I started with. If not, I've entered something wrong somewhere. But in this case it does return 0.447. Next I select the form I want. In this case G5 looks closest. G7 is for VLD shapes. I get back 0.228. So the G5 BC for this bullet is 0.228. Now I can go back to the first trajectory calculator and set it to work with G5 BC's and enter .228 and get a more accurate trajectory table than I would with .447 and the G1 BC.


*U.S. Army standard meteorological conditions (abbreviated, Std. Metro.), are often used as the standard sea level conditions. They are: 29.53 inches mercury (14.504 psi), 59F, and 78% relative humidity.
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Last edited by unclenick; 03-07-2011 at 08:02 AM. Reason: This is a work in progress
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Old 10-31-2011, 03:19 PM
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Ballistic coefficients reported by different bullet manufacturers may not be comparable due to the method of calculation and the accuracy of reporting. So BCs should only be used as a reference point and actual shooting should be done to determine your bullet trajectory under your conditions and velocities.
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Old 10-31-2011, 07:05 PM
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If you read Bryan Litz's book you'll find he's got actual measured BC's at different velocities for quite a number of bullets (I think he's got about 240 some in the newer edition), giving you different brands measured by the same equipment and operator under the same conditions so they are comparable. Many manuals only list a single G1 BC for a typical muzzle velocity, though Sierra tries to give the G1's at different velocity ranges and theirs are measured in their 300 yard indoor range. Some makers just estimate them. Litz has both G1's and G7's listed for each bullet in his book. Outside of Litz's book, I believe Berger is still the only maker listing both G1 and G7 BC's for each of their bullets.

If you have two chronographs or the Oehler Ballistic Lab with the microphone target accessory to get transit times, you can also measure your own BC's. The JBM on-line calculators will let you figure them for a number of BRL standard drag functions based either on velocity loss between two points or on transit time between points.

With an accurate chronograph and accurate BC information you can quite accurately predict come-ups on a computer. But you do need both of those things to do it.
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Old 11-02-2011, 06:25 PM
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Nick Thanks for repectfully pointing me to a information source I did not know of. Bryan Litz's 2nd edition book and software can be bought at: http://www.appliedballisticsllc.com/...files/Book.htm

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Old 11-03-2011, 12:50 PM
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Glad that helps. I should point out his program that comes on the CD that comes with the book is the same one you can download free at Berger's site except Litz adds a stability factor calculator. Nonetheless, the Berger version gives you a way to try it out. Also note that all Berger's BC's are now measured by Litz as he has become Berger's in-house ballistician.
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Old 03-12-2012, 08:10 AM
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This is an area which I really am "under" informed in, as you will probably determine very quickly...

But, is is safe to (logically) assume, that...
If a Bullet is very streamed lined, like a slender pointed (Spear) tip and has a Boat Tail,
that it will shoot flatter and longer distance than one which is blunt (FN/Wadcutter) ?
(assuming all else (diameter/muzzle vel/weight/environmental) are the same.

This works for just about everything else, but I thought I'd just ask.
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Old 03-12-2012, 04:44 PM
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Wikipedia has a nice tome, a basic formula that may or may not freak you out depending on your math background, but which is your basic add/ subtract, multiply/divide thingy...lots of definitions and other links that will give you a nice ride and should fulfill your needs.

Basically BC is just a number that indicates how "slick and pointy" a bullet is ...how easy it overcomes air resistance. Hi BC-are long, slender and pointy, slips smoothly through the air...low BC are short, fat with a flat nose...a flying beer cans.
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Old 03-12-2012, 06:14 PM
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Quote:
Originally Posted by Scotsman View Post
But, is is safe to (logically) assume, that...
If a Bullet is very streamed lined, like a slender pointed (Spear) tip and has a Boat Tail, that it will shoot flatter and longer distance than one which is blunt (FN/Wadcutter) ?
(assuming all else (diameter/muzzle vel/weight/environmental) are the same.
Yep. That's basically how it works. The longest, pointiest, most streamlined bullets are called VLD (Very Low Drag) designs. The more pointy and streamlined the shape, as Bryan Litz puts it, the better it slices through air. If you want to know how well it slices through air, relative to a standard projectile, the ballistic coefficient tells you that. Higher is better.
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Old 03-12-2012, 09:23 PM
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NFG, Nick, thank both of you for your prompt and informative replies.

I thought as much, but always like to discuss issues when I can.

I have bought a very basic "Rock Chucker" reload press and some other supplies, which I have squeezed a few rounds together already. Part of that "other" are a couple Books on recommended weight/powder combinations.

One thing that actually prompted my original question, is realizing that one can overdo the load quite easily, and there's no telling where that Round will go, regardless of how pointy it is.

My original thought was...
If it moves faster, it will not drop as much, and be more accurate.

HA, that's not the case, I found out !

My next question is...
Why isn't that true. Just what causes that instability with the increase of speed ?
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Old 03-13-2012, 06:55 AM
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Stability actually increases with speed slightly, but only up to the point other issues start to sabotage the effort. If you spin a bullet fast enough, one problem you get is wobble in flight. This is where mass symmetry imperfections are spun so fast that centrifugal effects cause the spin to become eccentric around the trajectory.

Another problem is core stripping. This happens inside the gun barrel, and occurs when the rotational acceleration of a bullet is so great that the jacket slips against the lead core as it accelerates. The bullet then exits with the jacket and core rotating at different speeds. As soon as they exit, friction causes their rotational speeds to equilibrate to a match. The core spins down up and the jacket spins up, but because the jacket has less mass, it loses more rotation than the core gains. At this point you have a bullet that's got speed bumps inside where the rifling pushed through the jacket and the cross section isn't very round, so again wobble gets into the picture and precision on the target deteriorates.

Finally, every once in awhile a long sleek bullet will actually disintegrate in flight. You can actually see a kind of gray streak when that happens in bright light. The eccentricity just grows by distorting the bullet until the stresses make it fly apart. This happens with long bullets in particular because long-for-weight bullets need the highest rate of spin to remain stable, subjecting them to higher centrifugal effects. You may have noticed before that long match bullets often come with a minimum rifling twist on the box, and this is why. The longer the bullet, the longer the lever arm drag has for overturning it, so the faster it has to spin to have the gyroscopic stability needed to prevent that occurring.

So, your first instinct is correct that fast shoots flatter. That's the reason for overbore cartridges like the .220 Swift. The Swift shoots the same bullet a lot faster than a .223, and therefore shoots farther more quickly and flatter. But the Swift also typically has only a 14" twist barrel instead of 7's, 8's, 9's and 12's that a .223 has. This prevents it over-spinning the bullet by the great acceleration it applies and causing the above problems, but it also means the bullet you use in it can't be too long or it won't stabilize adequately. You get a fast, flat shooter, but your bullet selection becomes limited to lighter bullets that are shorter and have lower BC's than the long ones the fast twist .223's can shoot. So it's a trade-off in capability that has advantage for the Swift only to limited distances. Beyond that, because the low drag bullets lose velocity in air more slowly than higher drag shapes do, they finally catch up to and pass the shorter, lighter bullets the .220 Swift is limited to. You just have to do more trajectory calculating to put those slower bullets where you want them. The longer it takes a bullet to make its journey, the farther gravity pulls it down.
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Last edited by unclenick; 05-18-2013 at 01:26 PM. Reason: typo fix
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Old 03-13-2012, 09:09 AM
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Nick, wow, what a very comprehensive reply !

If I may comment on this succintly...

"If you spin a bullet fast enough, one problem you get is wobble in flight."
I understand that. Much like a Speed Rating/Balance for an Automobile Tire. If you exceed that, you cause vibrations and instability's.

"the jacket and core rotating at different speeds".
Wouldn't this imply that a "Cast" Bullet would be better? One which does not have multiple metals in it. Or is that not done? Do all Bullets have more than one type metal in their construction?

"long match bullets often come with a minimum rifling twist on the box".
Just Match Bullets? I think it would be very advantageous to a Loader, for the manufacturer of those products to list that information on ALL Bullets! Additionally, why not list the FPS it can handle as well?

"the Swift also typically has only a 14" twist barrel instead of 7's, 8's, 9's and 12's that a .223"
I think this is the most interesting statement of all. Would this imply, that a Firearms Manufacturer actually sets up a condition via the size, length and twist of a Barrel, for only one type, weight, speed and rotation of round, for a given Firearm? Then subsequently, varies the specifications on the Barrels of it's products, to accommodate the application of a given Firearm.
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Last edited by Scotsman; 03-14-2012 at 08:44 AM. Reason: improper termonology
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Old 03-14-2012, 09:59 AM
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Quote:
Originally Posted by Scotsman View Post

"the jacket and core rotating at different speeds".
Wouldn't this imply that a "Cast" Bullet would be better? One which does not have multiple metals in it. Or is that not done? Do all Bullets have more than one type metal in their construction?
It's just peculiar to cup an core construction. You can, of course, shoot cast bullets, though they are not normally as hard as copper and it takes some skill and know how to load them to jacketed bullet velocities without leading or other problems. You can shoot solid copper (e.g., Barnes and Hornady GMX) without that issue arising, and I'm sure bonded core bullets withstand higher torque than the standard press fit designs do.

Quote:
Originally Posted by Scotsman View Post
"long match bullets often come with a minimum rifling twist on the box".
Just Match Bullets? I think it would be very advantageous to a Loader, for the manufacturer of those products to list that information on ALL Bullets! Additionally, why not list the FPS it can handle as well?
Some do list suggested twists, but you can use the free JBM Ballistics site stability estimator for yourself, as well. You need the bullet length for that, but they have a growing database of lengths, here. The calculator is good because you can adjust the muzzle velocity and weight and length and twist and air temperature and barometric pressure and see how all these factors interact to affect stability. That's an education in itself.

The FPS the bullet can be driven to without core stripping or disintegration depends on the rate of twist of the bore and the length of the barrel and the pressure curve of the powder you use. Too many possible combinations for a definitive single answer. The greatest acceleration of the bullet takes place during peak pressure which occurs when the bullet is still in the first couple of inches of travel, so that's where stripping initiates, well before final velocity is reached.

Quote:
Originally Posted by Scotsman View Post
"the Swift also typically has only a 14" twist barrel instead of 7's, 8's, 9's and 12's that a .223"
I think this is the most interesting statement of all. Would this imply, that a Firearms Manufacturer actually sets up a condition via the size, length and twist of a Barrel, for only one type, weight, speed and rotation of round, for a given Firearm? Then subsequently, varies the specifications on the Barrels of it's products, to accommodate the application of a given Firearm.
They usually try to find a compromise for a range of bullets and accuracy requirements. For .223 Remington/5.5645 NATO cartridges, the 7" twist now in military barrels is chosen to stabilize long, light tracer rounds. All other military bullets are spun unnecessarily fast by it, but the AR platforms aren't loaded to super high velocities like a .220 Swift, so it apparently hasn't caused a problem for military accuracy requirements (which allow something like 7" groups at 100 yards before a barrel is rejected and replaced).

The 7", 7",and 8" twist commercial barrels are mainly geared toward match shooters who will mostly shoot long boattail bullets in the 69-80 grain range, with an occasional 90 grain bullet shooter squeezing in. They can shoot a 53 grain flat base MatchKing just fine for standing offhand and sitting rapid, where accuracy requirements aren't as high.

The 9" twist is a compromise for folks shooting a range of bullet sizes ranging from light bullets to some stubbier match bullet shapes, but many match bullets don't stabilize well enough in them. The old 12" twist was always for lighter bullet weights that preceded the availability of long match .224" diameter bullets. It started with the original AR development which used a 14" twist for 50 grain bullets, like the .222 Remington, but it was changed after testing found those bullets weren't adequately stable in extremely cold air, so it was changed to 12" for early AR production.
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Last edited by unclenick; 03-14-2012 at 10:03 AM.
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Old 03-14-2012, 10:51 AM
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Nick,

THANK YOU !, for those links. Specially the "calculator" type. With my math skills, I need all the help I can get !

It's becoming very apparent, that one needs to be aware of just what he's shooting. Knowing specifications like Twist and Length of Barrel, are a great deal more important, than I once thought.

I was looking at some 357 Mag specs at Buffalo Bore at...
Heavy 357 Magnum Pistol & Handgun Ammunition
and FINALLY noticed that there was a VAST difference in MVel, given which length Barrel one would use. But, I'm still bewildered at their implied difference between a Pistol and a Hand Gun. There's SO much more I need to know about this.

Just to make sure...
I've seen several "opinions" offered, as to just how to measure a Barrel Length.
Should that measurement include the Chamber, or, from the front of a loaded Cartridge to the front end of the Barrel (excluding the chamber) ?

On the Twist of a Barrel...
I have rarely seen information regarding this. It seems like most Manufacturers don't regard it as being very important for the Customer to know. I'm finding that's definitely not the case. I will attempt to research the Firearms I have, and try to come up with that.

However, in the mean time...
IF I can't determine from any resource, what that given Twist is, what is the best way for me to check that on a given Firearm.

Nick, I can't tell you how appreciative I am, for your help with all this, and am very glad this Forum has an edit feature.
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Old 03-14-2012, 06:30 PM
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You are welcome.

These days Handgun is the generic umbrella term to distinguish handguns from long guns. Pistol refers to pretty much all handguns that are not revolvers, and revolvers get their own category. In old times the revolver gunslingers were called pistolero's so I think this use of the terminology is modern only and perhaps is English only, but don't know that for sure.

Barrel lengths of all kinds except revolver barrels are measured from the breech face of the gun to the muzzle, not including flash hiders, suppressors or other non-rifled extensions. For revolvers the barrel starts at the forcing cone which begins right in front of the cylinder where there is normally a few thousandths of an inch gap, and is measured from there to the muzzle. So, for a rifle or pistol you close the action and drop a cleaning rod in and measure how far it sticks in. For a revolver you open the cylinder, put something flat against the back of the barrel, then drop the rod in.

Many makers do actually publish barrel twist on their web site. You weren't looking before, but you'll start finding them now.

You can measure twist by pushing a tight dry patch through a barrel and watching how far it rotates over a given distance and figure out how far one turn takes or would take if the barrel were long enough. Inches per turn is the standard unit for rifling pitch in this country.
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"First contemplation of the problems of Interior Ballistics gives the impression that they should yield rather easily to relatively simple methods of analysis. Further study shows the subject to be of almost unbelievable complexity." Homer Powley

Last edited by unclenick; 03-18-2012 at 01:29 PM. Reason: Typo fixes and added information.
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Old 03-18-2012, 01:42 PM
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Note: Moved Scot's other questions to new thread in handloading, here.
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"First contemplation of the problems of Interior Ballistics gives the impression that they should yield rather easily to relatively simple methods of analysis. Further study shows the subject to be of almost unbelievable complexity." Homer Powley
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Old 09-18-2013, 07:56 AM
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Truth in Published B.C. by major bullet manufacturers ?

I shoot 7mm and I have noticed
some hi B.C. claims by some mfgrs on some bullets that seem all out of proportion
with the claims of competitors bullets and I wonder if any of the mfgrs are
developing a reputation of stretching their B.C. performance optimistically ?
I look at Barnes, Speer, Sierra and Berger mainly. 140 to 168gr bullets.
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Old 09-18-2013, 04:08 PM
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Get a copy of Bryan Litz's book, Applied Ballistics for Long Range Shooting. He has tested something like 250 bullets all by the same method to get a valid comparison. It is certainly the case that many published single number ballistic coefficients are valid only at some muzzle velocity. Some pick a velocity higher than others, making their bullets look better than they actually are. It is a result of believing that many shooters, given an array of similar choices, will just pick the bullet with the highest BC number. That's probably so for some.
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"First contemplation of the problems of Interior Ballistics gives the impression that they should yield rather easily to relatively simple methods of analysis. Further study shows the subject to be of almost unbelievable complexity." Homer Powley
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Old 09-19-2013, 12:44 AM
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Quote:
Originally Posted by unclenick View Post
Get a copy of Bryan Litz's book, Applied Ballistics for Long Range Shooting. He has tested something like 250 bullets .

I am sure it is a good book but I aint paying Fifty bucks to get the truthful data that
the manufacturers should be supplying as description of their products. I realize he needs
to be paid for his work in uncovering the virtual fraud that is taking place in the
bullet industry but I think the mfgrs should be paying him.

Take a look at some samples of his work re 7mm bullets from Hornady and Sierra
The yellow highlighted numbers are his real world tested data and the G1 column
are what the manufacturers for the most part are using. HORRIBLY EXAGGERATED !
Attached Thumbnails
Ballistic Coefficient Basics-_1hornady.jpg   Ballistic Coefficient Basics-_1sierra-7mm.jpg  
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Old 09-19-2013, 05:10 AM
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Well, yes and no. What many shooters don't realize is that BCs can change dramatically just with the load data and individual barrel. I think that Rick Jamison did an article in Shooting Times many years ago, shooting a number of loads of one bullet in the same barrel and measuring the downrange velocities.

It varied much more than you'd think.

So.... with a long heavy barrel on a rigid test gun in a wind tunnel with perfectly tuned loads, they might well get much better numbers than a random sporter barrel out in the wind with who knows what load.

I'm not saying they are not exaggerating - I'm saying there is a lot more tolerance in these numbers than you'd think.
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Old 09-19-2013, 08:19 AM
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Also "G1" and "G7" are two different 'models', with different assumptions.
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