Author Topic: Explanation of Sectional Density and Ballistic Coefficient (Read 1602 times)

billjoe · « **on:** February 27, 2003, 07:26:20 AM »

Can a person judge a hunting bullet by sectional density and ballistic coeffient ratings?
In simple language, the density refers to the hardness and the BC to the way it will fly or stay airborne if you will. Correct? So if a bullet has a higher BC than another, it will fly farther and have a lower tragectory than the other bullet? And a higher SD will penetrate deeper, but expand less than a bullet with a lower number?

Is this close to how it works?
And can this information tell you which would be more likey good to use in a contender for hunting?

bj :?

RonF · « **Reply #1 on:** February 27, 2003, 07:39:41 AM »

I don't consider myself an expert here, but I'll give this a try. First, sectional density is simply a measure of the weight of the bullet compared to its cross sectional area. For a given bullet diameter, a heavier bullet has a higher sectional density than a lighter one. It has nothing to do with hardness. In fact, for a particular bullet shape, diameter and length, a soft lead bullet will have a higher sectional density than a copper jacketed or hard cast bullet because the soft lead is heavier. You are close on ballistic coefficient. It's related both to shape and weight. It determines how a bullet will hold velocity after firing, i.e. it determines trajectory once velocity is set. Whether or not a bullet performs in a Contender depends more on the bullet design and construction than on either its sectional density or ballistic coefficient, although generally, given similar construction, bullets of higher sectional density will penetrate deeper if driven to similar velocities. Hope I haven't confused you. Just about any good reloading manual will have some information and discussion of these things - I think the Sierra manual is especially complete.

RonF

Paul H · « **Reply #2 on:** February 27, 2003, 07:47:10 AM »

Sectional density is a meaningless number. Folks like to use it to try and prove that small caliber rounds with heavy for caliber bullets will perform the same as larger caliber rounds. Others try and use it to predict the performance of expanding bullets on game. The reason the number is meaningless is that it doesn't take into account a bullets construction.

Balistic coefficient tells you how aerodynamic a bullet is, which will allow you to calculate the rate at which a bullet looses velocity, and hence calculate it's trajectory. One issue not often considered is that the BC will change depending on the atmospheric conditions, air pressure and humidity.

helobill · « **Reply #3 on:** February 27, 2003, 09:28:03 AM »

Ballistic coefficient also changes with velocity (mostly due to hypersonic speeds of most bullets). Sierra's web site lists velocity range and BC for their bullets. www.sierrabullets.com
Helicopter Bill

PaulS · « **Reply #4 on:** February 27, 2003, 09:29:10 PM »

Quote from: helobill

Ballistic coefficient also changes with velocity (mostly due to hypersonic speeds of most bullets). Sierra's web site lists velocity range and BC for their bullets. www.sierrabullets.com
Helicopter Bill

HeloBill,
You are under a mistaken assumption about ballistic coefficient being variable. The air density is the variable and it changes with the velocity. I know that some manufacturers list multiple BC's to compensate for the variable density of air at sub-sonic, super-sonic, and hypersonic speeds but if the variable is placed in the ballistic calculations for the changing density of the atmosphere one BC is as accurate as any weapon / shooter. I have proven this with my own ballistics program and tests at varying ranges with multiple weapons.

PaulS

RonF · « **Reply #5 on:** February 28, 2003, 01:43:09 AM »

Now wait a minute, Paul. The air density is a property of the air, not whether I fire my .45 Colt or my .22-250. You may be using a variable air density as a model of what happens, but the air density is the air density as determined by barometric pressure and relative humidity.

RonF

helobill · « **Reply #6 on:** February 28, 2003, 04:11:55 AM »

Yep Ron I think you're right. sub sonic vs trans sonic vs supersonic flight , speed of sound on a standard day is 1116 fps and the coefficient of drag changes as you go through sub/trans/super sonic flow. An object moving slower than the speed of sound is subsonic, where most compressibility effects are small and can be neglected. If the object moves near the speed of sound, conditions are said to be transonic, and other compressibility effects (like flow choking) become important. If an object moves up to four times faster than the speed of sound (and I don't know of any standard bullets that exceed 4400fps although some are getting close like the 17rem at 4100fps), the conditions are said to be supersonic, and compressibility effects (like shock waves) are present in the gas. So the total drag on a bullet will change as its speed changes, same as an aircraft wing or Helicopter rotor (that's one of the reasons I can't exceed the speed of sound on a propeller or rotor, and the reason aircraft designed to fly above Mach 1 have a different shape airfoil that is inefficient at supsonic speeds). Air density changes with temperature, pressure, and humidity and is constant for a given set of conditions. By changing this constant in your ballistic calculations you are compensating for the changes in flow at different speeds. It is not a varying density but a varying compressibility that effects drag coefficient (and therefore BC) that is changing with the changing speed. Guess all that Aero they forced into me in Naval Flight Training is finally worth something.

Since BC will be different it is important to know where it is starting, especially if you intend to use a bullet at a speed significantly different than the speed used to determine the BC.
Helicopter Bill

RonF · « **Reply #7 on:** February 28, 2003, 07:25:21 AM »

Exactly, helobill. There are lots of ways to model something mathematically and have it agree with observations pretty closely. The models are not causal and just because one works doesn't mean that's what is happening physically. Paul has used one method, and Sierra another. Both may work - in fact, both probably work really well for what shooters need - and neither may be a good explanation for what actually happens!

RonF

PaulS · « **Reply #8 on:** February 28, 2003, 05:09:01 PM »

Ok boys,
At sub-sonic speeds air behaves as a gas - it compresses and flows but at supersonic speeds it behaves as a liquid - it no longer flows or compresses. That is why supersonic airfoil leading edges are angular (wedge shaped) and the Cesna has a curved surface. At hypersonic speeds, air is so dense that the compression wave (out in front of your bullet) causes heat to transmit (infra-red radiation) to the points nearest their apex.
The bullet in flight is launched at say 3800 FPS (hypersonic) a SPIRE point bullet will have less drag than a spitzer or round nose as long as the speed stays above mach 3. As the bullet makes the transition to supersonic the spire points three waves lose their separation and attach to the angular sections. The spitzer bullet goes through a similar transition it only has two shock waves with a turbulent wave between them. As the pressure wave attach some of the turbulence is lost and a third shock wave forms at the cylindrical / ogive junction of the bullet. The round nose has a similar condition to the spitzer but the wave is more pronounced at the nose. As they reach subsonic speeds the spire point has more drag than the spitzer due to the turbulence where the conical nose meets the body and the turbulence at it's base. The spitzer really shines with air flow "laminar" all the way back to it's base where turbulence begins. The round nose has "laminar" flow at lower speeds than the spitzer and performs better at about 600 fps than the others.

If you don't think air density increases with velocity try sticking your arm out the window of a car at 100 mph. The relative density of air is the reason that the drag coefficient varies at different velocities.

PaulS

RonF · « **Reply #9 on:** March 01, 2003, 02:12:15 AM »

No sense arguing this any more if liquid doesn't flow and we're going to use 150 fps examples (the car) to argue supersonic effects. Guess I won't be able to pour my beer the next time the flight attendant hands me one....

RonF

helobill · « **Reply #10 on:** March 01, 2003, 03:13:07 AM »

Yeah Ron, guess you're right. Last thing I'll say is the change in density he is talking about is the result of the shock wave not the other way around, you can model the actual changes in flow due to shock wave effect, etc or account for it by using a changing density. And hypersonic flow doesn't start until approximatelly Mach 4-5 (there is no one speed where this happens, it is situation dependent) and bullets aren't there yet.
nuf said.
Bill

bsekf · « **Reply #11 on:** March 01, 2003, 05:09:45 AM »

OK guys, now for the rest of us poor souls. I gather that SD of a bullet is not as important as BC, right? OK, which is better a high BC or a low? and why? and please remember KISS, for the poor souls.:wink:

RonF · « **Reply #12 on:** March 01, 2003, 08:34:34 AM »

SD is ONE measure of how a bullet might penetrate, but it's not as important as bullet construction. BC tells you something about how a bullet maintains its velocity, and thus allows predictions of trajectory. A high BC is better, as it means the bullets shoots flatter for any given velocity. Hope this helps.

RonF

TopGun · « **Reply #13 on:** March 01, 2003, 02:12:56 PM »

Now that we have that cleared up, is there a physics major out there that will explain the differences between momentum, kinetic energy and the theory of relativity? I'm not sure, but IMHO, I think they are all an enigma, wrapped in a mystery. In order to understand these things, one must pour three glasses of 'Jack' :grin: , one with no ice, one with 2 cubes, and one with 3 cubes and 2 fingers of water. Now, sit back and start a fire, chug the 'no ice', drink the 2 cube, and start sipping the 3 cube and 2 fingers. Now sit back farther, look at your fire, leave your guns alone, go to sleep and and dream of that big buck you missed last season because he was too far for the bullet you used. You now realize should have selected a boat-tail instead of a round-nose of the same weight.

When you wake up you will have a head-ache, but not from the missed deer, or trying to understand all of this. :? It will be from the 'Jack'!

helobill · « **Reply #14 on:** March 01, 2003, 05:23:52 PM »

TopGun,
Hope you don't mind if I substitute Crown for Jack :lol: . Thinking back 20 years to flight school caused me to use a 12 pack to cool all that wood between my ears back down below flash point.

(not that that was a bad thing mind you.)
Helicopter Bill

You must have a firm belief that you know why helicopters fly, for if you stop believing they stop flying.