Friday, June 6, 2014

Gravity, Velocity, and Wind

Part of what you need to be an accurate rifle shooter is knowledge. The following are a couple of the major considerations when you think about what the bullet is doing after it leaves the barrel.

The first is gravity. When the bullet leaves the barrel it begins falling. It is subject to the effect of gravity just the same as a bullet you drop out of your hand. If your barrel was set parallel to the ground and you could drop one bullet at the exact same instant the other bullet left the barrel traveling 2700 feet per second, they would hit the ground at the same time. There is no such thing as a "flat shooting" round. All of them fall and accelerate in that fall at the exact same rate.

To compensate for that, gun and rifle sights are mounted so that the barrel is tipped up to a greater or lesser extent depending on the nominal range the weapon is designed for. If you successfully "zero" a rifle at 200 yards, you are compensating for that gravitational effect for the particular velocity of the round you are using at that particular distance by changing the angle of the barrel. The bullet will rise for part of the distance and then fall back down to the target. The amount of rise and fall is a function of time, so the velocity the round is traveling when it leaves the barrel matters.

Making velocity the next consideration. The faster the bullet is traveling, the further it will go in the same amount of time. A bullet may leave the barrel at 3000 feet per second, but it immediately starts slowing down. It continues to lose velocity all the way to the target. The rate it loses speed is related to it's ballistic coefficient, a measurement of how aerodynamic the bullet is. This is what can be changed. A very high ballistic coefficient coupled with a high muzzle velocity means the bullet travels further in the same amount of time. This can be calculated and there are software programs that help with the complex math involved. There are even online calculators.

Let's take one common example, 5.56 NATO. Say I zero my rifle at 180 yards.  At 2900 feet per second, the bullet leaves the barrel, 2 inches below the sights. It rises due to the angle of the barrel, passing through the aiming point of the sights at approximately 46 yards. It continues to rise until it is about an inch and a half above my aiming point at about 110 yards, then begins to fall. It passes back through my point of aim at 180 yards and is 2 inches below my point of aim at 200 yards.

As you move back to longer ranges, the arc the bullet travels though becomes more pronounced. At 500 yards, your bullet will rise a couple of feet before dropping back in. It takes .6 seconds for that bullet to travel 500 yards, and when it arrives, it's only going about 1600 feet per second. You make this compensation with your sights, usually adjusting the rear sight a set number of clicks for every 100 yards further back you move. When you look through your sights, you keep the sight picture the same, so what you are doing is increasing the amount you tip up the front of the barrel, even though that is not how we usually think about it.

One piece of trivia. When the French Army sighted their rifles in at what we would call "battle zero", it was done by shooting at a white square of paper on a target backer. Whatever the weapon, the distance that put a hit on that paper square was the "point blanc". We made it point blank in English. For a military rifle, point blank range is nominally about 200 yards. Since a military shooter only needs to be able to hit a torso, elevation differences inside of point blank range are immaterial. Targets further away require sight adjustments. When I hear about someone being shot at point blank range, I always think that is pretty good shooting.
Update: From the comments, Nosmo King offers a far better definition of point blank range:
the maximum distance at which firing at a center aiming point on a target of particular size will result in a bullet strike on that target at any distance from the muzzle to the point blank range with a particular firearm using particular ammunition, absent atmosperic induced deviation.

This is all background, the beginning of the information you need to know to shoot a rifle accurately and be able to adjust your sights for various distances. We could talk about wind, wind direction, and wind differences 300 yards out from where you are set up. Just an 8 mph crosswind will push that same .223 bullet almost 2 and a half feet off it's point of aim at 500 yards.

And we have only considered one cartridge, with one bullet, and one velocity.


Stackz O Magz said...

Great post! I just did an A2 Irons refresher last week for the AR on my page. I'm amazed the amount of people that do not know these things.

Goober said...

There is no such thing as a "flat shooting" round.

This is not true, by the definition of what most people think when they say it.

The reason is because the bullet is definitely falling to earth just as fast as any bullet, but the bullet out of a “flat shooting” gun is traveling faster, so it goes farther in the same fall. If you look at that plotted on a graph, it is, indeed, a “flatter” trajectory.

A slower bullet, to go the same distance, will need to be “lobbed” more, and therefore will have a less flat trajectory. A faster bullet will not need as much “lob” to go the same distance. A .30-06 is “flatter” shooting than a 375 H&H, which is “flatter” shooting than a 45-70.

As for your “zero” distance, I always explain it to people in much the same way you just did – you sight a rifle in at 200 yards, and essentially for anything inside 350, you will kill your target by holding the sights on where you want to hit him and he will die. At 350, your hit point will be 8” low with most hunting grade calibers, but that’s good enough for combat and for hunting. That’s why your zero point is so important. I zero my rifle at 200 yards so that I can hold on target and shoot for any “up close” work, and anything further away, you’ll generally have enough time to range it and think it through.

But that’s the most important part about shooting, is knowing your rifle, and knowing your round. How much will it drop at 500 yards? If you don’t know, you’ve no business firing it out that far, because you WILL miss

ASM826 said...


There is "flatter shooting". As you mention, if you send a projectile zipping out at 4000 fps, it will travel farther in the same amount of time, meaning it drops less. Where this starts to be problematic for people is when they misinterpret "flatter" for "flat". Because the longer the range, the more it slows, the longer it's been in the air, and the more it drops, no matter the starting velocity.

You comment, "Know your rifle" is so important, I think that will be my next post. Thanks, ASM826

Nosmo King said...

If I may be so presumptious, may I offer another defintion of "point blank range"?

Point blank range is the maximum distance at which firing at a center aiming point on a target of particular size will result in a bullet strike on that target at any distance from the muzzle to the point blank range with a particular firearm using particular ammunition, absent atmosperic induced deviation.

Point blank range will vary based on projectile velocity and ballistic coefficient, target size and winds.

ASM826 said...

Nosmo King,

Yes, that is a far better explanation/definition of "point blank". This is exactly the type of comment and discussion I had hoped these posts would generate. I updated the original to include your words.


Borepatch said...

What a great discussion here in the comments.

Comrade Misfit said...

There is also "sectional density", which is why a 16" gun could throw a projectile out past 40,000 yards at roughly the same initial muzzle velocity of a .30-40 Krag.

Borepatch said...

I *so* want to shoot a 16" gun at a blog shoot ...

Goober said...

Comrade - good point. It's another thing that I've tried to explain to folks when I'm out teaching them how to shoot far.

A .223 round and a 300 win mag round both leave the barrel at 2900 feet per second, but that 300 win mag round will go further than the 223 round for one simple reason:

It has taken more energy to accelerate the larger mass to that velocity, so the larger bullet carries that much more energy upon leaving the muzzle.

This is why snipers in our armed forces do not generally use the general issue 5.56 NATO round, but rather start with the 7.62 NATO, and then move up to the .338 Lapua and the BMG 50 for the longest shots.

It's also why you see big rifle calibers in sniper rifles, like the 40 plus caliber rifles the brits are shooting, and of course, the barret 50. You don't need a BMG 50 to kill a dude, but if you want to do it out past 1,000 yards you do. (a 300 win mag hits with about the same oomph as a 45 ACP at 1,000 yards, which, if you know anything about guns, you know that's really not that much oomph - pistols are super wimpy).

Interesting point - the barret 50's muzzle velocity is about the same as a .30-06. But try shooting a .30-06 at 1,500 yards and get back to me...

Sectional density for long range shooting is arguably more important even than ballistic coefficient.

Nosmo King said...


"atmospheric," not "atmosperic."

Need to hire a new proofreader.....