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bitmasher's picture
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Why a bullet shoots flat or doesn't. Ballistics 101

I want to dispel some mysteries about "flat shooting" with this thread and hopefully to elevate the knowledge of basic ballistics. I suspect the info below will be pretty simple to some, but to others "flat shooting" is a mystery.

How flat a rifle will shoot depends on two and only two variables (assuming it is functioning properly). These are:

- The ballistic coefficient (BC or sometimes C) of the bullet
- The velocity of the bullet

The ballistic coefficient is a measure of drag or how well the bullet slips through the air. The higher the BC the less horizontal velocity a bullet will lose during its flight. Therefore a higher BC bullet shoots "flatter" that is to say it covers more horizontal distance in the same time frame as a lower BC bullet (assuming equal inital velocities).

A BC of 1 is a perfect bullet, because it loses no energy to wind drag (resistance). A BC of 0 is a terrible bullet, it comes out of the barrel and just drops to the ground, it would be the case where the bullet immediately dumps all its energy to the surrounding air as heat (and itself what be quite hot even if it held together). A BC greater than 1 is impossible, that would suggest that somehow the bullet was gaining energy merely by flying. By the way, BC is earth atmosphere realitive. BC's across the boards would go up if we were having a shoot on mars or the moon. Why? Less atmosphere to contend with and incidentally that does suggest that the 270 would be a more lethal elk round at a greater distance if we were chasing bulls on the moon.

Either BC (0 or 1) is unrealistic and most coefficients are in the area of .3 or so. But it is good to understand what happens in extreme cases.

Now velocity. If two bullets of the same mass and same BC are shot at the same angle to the ground, the flatter shooting will be that which has a higher velocity. This is because all bodies fall to the earth at the same rate regardless of mass. It is one of newton's laws and galileo demonstrated it by dropping lead balls off of galloping horses. Its not intuitive but true.

So the faster bullet covers more horizontal distance with the same drop as a slower bullet. The flatness of cartridge or rifle is measured in how much drop per distance covered. If you cover that distance faster you will drop less because the time interval between markers is less.

Knowing this it is possible to see why some people say a 243 or a 270 is flatter shooting than other rifles. This is simply because these cartridges tend to push the bullets at faster rates compared to other rounds (like the 30-06 or even worse the 444 marlin). However by careful selection of bullets and mass it is possible to create a 30-06 that shoots as flat as a 270.

Well I hope some find this useful. Feel free to point out any errors or omissions.

BTW, Expatriate mentioned basically the same thing to Lever awhile back, in fewer words over a couple of posts. Here it is if you would like to read ballistics from someone elses perspective.

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Why a bullet shoots flat or doesn't. Ballistics 101

Flatter doesn't mean more accurate, a 270 might have 2 inches less drop than a 30-06 at 300 or 400 yards, but its not any more accurate. Bullets that have the boat tail design, like many Hornady bullets do, these typically perform better at long range than the flat-base bullets. Velocity has a little to do with it, ut of course, the faster a bullet exits, the more air resistance it meets. So many factors, so little room.

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Why a bullet shoots flat or doesn't. Ballistics 101

Well said. Ballistic coefficient directly influences the rate at which a projectile loses velocity. The higher the BC, the more velocity is retained over distance. The flattest rounds are ones that start fast and have a bullet with a high BC to maintain that velocity.

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Why a bullet shoots flat or doesn't. Ballistics 101

Excellent presentation there bitmasher!!!

It reminded me of a simple but effective Newtons Law lesson that my science teacher taught us in the 9th grade that has stuck with me and might help some remember this section of your 101 more easily.

"If you were to set up a rifle, at any give height, so that the chamber and the muzzle were perfectly level, take two bullets, one just the bullet and the other still attached to the cartridge, set up a device at the end of the rifle barrel to hold the bullet without the cartridge, have that device set so that the bullet fired from the rifle upon exiting the muzzle would trigger the release of the device holding the bullet, both bullets would hit the earth at exactly the same time, one at the end of the barrel, the other some distance away that would be determined by the velocity and atmospheric resistance of the fired bullet. Velocity and atmospheric resistance would only affect distance as related to the speed that the bullet could travel in the amount of time that it would take gravity to pull it to the earth, which is the same for both bullets at 32feet per second per second (no it is not a misprint there are 2 per seconds in acceleration in newtons law)."

This is taken word for word from my old 9th grade science folder.

[ This Message was edited by: JTapia on 2004-02-06 18:27 ]

bitmasher's picture
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Why a bullet shoots flat or doesn't. Ballistics 101

I'm impressed, I doubt I could even easily find my highschool diploma let alone recall a lesson from ninth grade. Young Jtapia must have really been tuned in when the teachers talk turned ballistics. Wink

Great example. Sometimes simply demonstrations can cut to the point when reams of words can't. Although Mister_V did a great job of cutting to the chase pretty quick.

I agree on the boat tails too CaptO. I suspect there is a good aerodynamic (fluid flow) reason why the rounded edge cause less perturbation in the flight path, perhaps it has to do with the "kick" of the air as channels around the end of the moving bullet. Fluid dynamics is great stuff, very intense though, with non-intuitive results. A simple one is that dimpled round balls fly farther than completely smooth balls.

It is true accuracy has nothing to do with bullet drop by the numbers. However I think Ex said it best (in the links) when he pointed out that the more drop you are getting per yard (near impact) the harder it is to be accurate in the "real world". The real world meaning the world where you can be off in your guess of distance by +/- 50 yards or more (assuming your shooting over hundreds of yards, if you are off by +/- 50 yards in your guess on a 70 yard shoot, might as well pack the bag and go home).

The more your off in your horizontal judgement the more drop is going to effect your ability to hit the boiler house consistently. A perfectly flat shot (a laser) being off 1 yard or a 1000 yards in your horizontal distance guess it would make no difference in your "real world accuracy" since there is no drop. (just for the wise guys, yes that is not true if your laser-buck-buster is going off near a black hole, but nobody i know chases deer in deep space. If that makes no sense, thanks to Einstein we know that light rays can be bent by powerful (read mind numbingly powerful) gravitational forces. This is why black holes are called "black", they are perfectly black because no light (in the visible spectrum) comes back out).

[ This Message was edited by: bitmasher on 2004-02-06 23:34 ]

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