Nothing pleases a shooter more than having conditions just right for that important shot, whether it is at game or a target. Most of the time, the situation is close enough to ideal that we can get the shot off without undue concern that factors outside our control will cause a miss. It seems worthwhile to consider those times when that isn't true, when unfamiliar factors work against us. Several sets of conditions fit into that category, among them wind, altitude, incline angle and gun position.
Wind is the most constant of the factors we are discussing. It seems to be always with us. For the hunter, it usually isn't so much of a problem as for the target shooter. Wind effects have been well understood for a long time by those studying ballistics, and can be easily grasped by the ordinary shooter.
Two characteristics of the wind are most important to consider, speed and direction.
The stronger the wind, from whatever direction, the more error it will cause. This can be very impressive. A .530 caliber round ball fired at 1800 fps at a target 100 yards away, with a 20 mph wind blowing constantly at right angles to the line of flight, will cause the ball to drift more than 29 inches. Reduce the wind speed to 10 mph and the deviation is much less, but still significant, almost 15 inches. Reduce it to 5 mph and the ball still drifts more than 7 inches away from the wind. In a rough correlation, doubling wind speed also doubles the error.
Wind may blow from any direction, but some are more troublesome for the shooter than others. First, a direct headwind or tailwind causes the least problem. A headwind has the effect of increasing the drag of the bullet, because drag goes up as velocity goes up, and a headwind has the effect of increasing velocity of the bullet relative to the air. This increased drag will slow the bullet's flight to the target a bit, relative to the ground, giving gravity more time to work, thus increasing the drop. So, a headwind causes the bullet to fall a bit low. The opposite is true for a tailwind, since that wind speed must be subtracted from the bullet's velocity relative to the air, which decreases the drag, increases the velocity relative to the ground, reduces time of flight to the target and thus the drop. So, bullets fired with a tailwind drop less than in calm conditions and strike a little high.
These effects are both very small in anything like reasonable conditions, and can be ignored for both headwinds and tailwinds.
Wind from any other direction will push the bullet sideways away from the wind, causing a deviation from the line of flight to right or left. For any given wind speed, the effect increases as the wind approaches a direct crosswind at 90 degrees to the line of flight. In other words, an angling crosswind causes less mischief than a direct crosswind. If the wind is a steady one, charts are available to calculate the effect, taking into account the strength and angle of the wind. That's possible because any crosswind can be broken down into two components or vectors, a crosswind and headwind/tailwind one. Target ranges sometimes provide gauges to monitor the wind so that these calculations can be made. The hunter must fall back on experience and take his best guess.
If you must have a wind, make it a steady one. Winds variable in speed and direction are hardest to figure and compensate for. We've all seen days when the wind is calm at the target but blowing at the muzzle, or when gusts are the order of the day. A wind which puffs at the muzzle, causing a slight error there, will result in a larger error at the target, because the bullet's track is permanently changed. Conversely, the same wind near the target causes less error, because the bullet will reach the target before much deviation can occur.
The shooter isn't totally helpless in the face of this unpredictable wind. Shooting in the wind is a learned skill, and with practice can be lived with. There are are also two decisions he can make which will help, before the shooting starts. Not all bullets are effected by the wind to the same degree. Let's consider large and small ones, and fast and slow ones.
Large, heavy bullets or balls have more inertia, that property which makes any moving object tend to continue moving in a straight line. Large, heavy bullets resist the sideways push of wind better than small, light ones.
Small bullets fly at greater velocities than large ones, generally. Shooting in the wind, fast is good. The ball reaches the target quicker, giving the wind less time to divert it from the straight and narrow.
So, the ideal bullet for shooting in the wind is a large, heavy, but fast one. We run against constraints of recoil, comfort and quality of shooting as the charges required to move a large bullet with high velocity increase, so a point will be reached where a compromise must be made. That point must be decided by each individual shooter.
Most of us shoot in our own territory the majority of the time, so that our altitude above sea level can be considered a constant and forgotten. But, what if we go on that dream hunting trip or competition shoot, and the site is greatly different in altitude? This can cause problems, and deserves some thought.
It is a simple and well known fact that air becomes thinner, less dense, as altitude increases above sea level. Thinner, in this sense, means fewer molecules of gas per unit of volume. This thinner air creates less drag for a flying bullet, because it has to push fewer molecules aside in order to make its way through the air. The effect this has on accuracy depends upon whether you are shooting at a higher or lower altitude than usual.
If you live near sea level and sight your gun to hit directly at point of aim there, then shoot it at, say, 10,000 feet altitude, the gun will shoot high. This is because the bullet is subjected to less drag by the thinner air, loses less velocity before reaching the target, reaches the target quicker, before gravity causes so much drop. For the .530 ball discussed above, shot at 100 yards, the error would be on the order of 1.5 inches.
It's easy to see that if the situation is reversed, so will the effect be. If you sight the gun in at 10,000 feet but then shoot it at sea level without making any adjustments, it will shoot low. The more dense air at sea level has more molecules per unit of volume, creates more drag on the bullet, slowing it down and allowing gravity to cause more drop before it reaches the target. The error would be of the same order as in the example above, somewhat less than 1.5 inches low.
Think uphill and downhill shooting. There is a built in error in accuracy when shooting either uphill or downhill at steep angles. This cannot be avoided, but can be compensated for. It affects all guns, black powder and smokeless. It isn't caused by misjudging the distance, it occurs even when shooting over carefully measured distances. The effect can be large enough to cause a miss on a fairly large target at longer ranges. The effect gets worse as velocity decreases and as distance to the target increases. It also gets worse as the 'incline angle', the steepness of the slope, increases. Presented with a shot up or down a very steep slope, with a slow moving bullet and at maximum range, we are in deep trouble.
In order to understand why this happens, we must go back to basic ballistics and some definitions. This might be a good time to read over the FAQ, Basic Ballistics.
When a gun is sighted in on the level, the Line of Sight goes from the shooter's eye, through both sights of the gun and straight to the center of the target. Because of gravity, and the drop it causes in the bullet once it leaves the muzzle, the barrel, or Line of Bore, cannot be aimed in the same way. It must be pointed above the target an amount exactly equal to the True Drop. That way, the bullet starts out flying upward, but falls exactly into the target just as it reaches target range.
It is always assumed that sighting in of a gun takes place on the level, and so the flight of the bullet, the Line of Sight and Line of Bore are considered to be essentially horizontal. True Drop takes place "straight down", and is calculated or measured vertically from the Line of Bore to the Path or Trajectory of the bullet. It is always assumed to take place at 90 degrees, a right angle, to the Line of Bore.
Consider the situation encountered when firing up a steep slope at exactly the same range at which the gun was sighted in on the level, say 100 yards. The Line of Sight is still aimed straight at the point of intended impact on the target. The Line of Bore is still aimed just enough above that point to cause the bullet to drop exactly onto it. But...and, here's the rub...True Drop no longer takes place at right angles to the Line of Bore. If it did, we would not miss. It still takes place vertically, because that's the only direction gravity works, but that causes it to be at a steep angle to the Line of Bore, which is tilted steeply up. At that angle, it would take more than normal drop for it to reach the normal Path of the bullet. However, as we shall see, Drop is the same as always, so the bullet winds up above the usual Path or trajectory. In other words, the gun shoots high.
Many people, when presented with this problem, conclude the bullet flies higher than normal because Drop is less than normal. Various explanations are offered as to why this should be true, but all conclude Drop is less. That is not true. The solutions offered are frequently well thought out, the mathematical 'proofs' are correct, but the conclusions are wrong because they begin with a faulty assumption. Drop cannot be anything other than exactly the same as when firing that gun on the level. The distance from muzzle to target is still 100 yards. The velocity of the bullet is still exactly the same, so that time of flight to the target is exactly the same. The acceleration of gravity is certainly always the same, since it is a physical and mathematical constant. A bullet free to fall, subjected to the acceleration of gravity for the same time, will fall the same amount. Drop is the same. The problem with conceptualizing it is in realizing the problem is the angle at which the drop occurs, not the amount of drop.
The same error occurs in shooting down an identical slope, and to the same degree. All guns shoot high when fired either up or down a steep slope.
As an example of the degree to which this can be a problem, that .530 ball at 1800 fps, sighted in for 100 yards on the level, but fired up or down a 65 degree slope at a target 100 yards away will shoot 5.6 inches high. At 200 yards, with the gun sighted for that range on the level, the error is 38 inches.
Few of us are presented with the problem in our normal shooting situations. Remember, though, that it works for all guns at all steep angles, and that short ranges reduce but do not eliminate the error. Squirrel hunters aiming nearly straight up at 20-30 yards should factor it in, and more squirrels will wind up in the pot.
The Lyman Black Powder Manual, 1975, has a thorough explanation and a good graphic which makes all this very clear.
Some ballistics puzzles are more fun to work with than others. I've always enjoyed playing mental games with the effect of varying gun positions on accuracy. These mental games can serve as a powerful learning tool, because they demand a thorough understanding of how our guns and their sights work, the relationship of these to the flight of the bullet, and the constant effect of gravity on our shooting. To some, such games seem frivolous, but I've noticed they can usually not answer the questions. Let's consider some fairly radical positions. Keep in mind that these are theoretically perfect games, which we could not, of course, duplicate exactly in real life. But, we could get close.
What happens if you shoot a gun with it turned directly on its side? You miss. Yes, but by how much, and in what direction? To answer that, we must describe the gun. In all these examples, we'll use the same .54 caliber rifle as above, with a muzzle velocity of 1800 fps and sighted to hit point of aim at 100 yards. Running this information through Gun Controller® by RSI, we find that True Drop at 100 yards is 9.75 inches. So, from our understanding of basic ballistics, we know that on that gun, when fired in the normal position, the sights are aimed straight at the center of the target, but that the bore cannot be. If it were, we would hit low, because gravity will cause the ball to fall all during its flight to the target. As a matter of fact, we would hit 9.75 inches low. So, to cancel out that drop, we aim the bore exactly 9.75 inches above the center of the target. Now, when we fire, the ball flies upward at first, then drops down onto the target, hitting the 'X', of course. All is well.
When we turn the gun on its side and shoot, using the same sights, these same facts hold true, but bring about different results. Say we turn the gun on its left side. Now, that compensation for drop which we so carefully set up will no longer work, because the bore is now pointed 9.75 inches to the left and level with, not above, the target. Since the compensation for drop has been eliminated, the ball will fall 9.75 inches below the point of aim. The end result will be that the ball will strike 9.75 inches to the left and 9.75 inches below the point of aim.
Turn the gun on its right side and the ball will strike 9.75 inches to the right and 9.75 inches below the point of aim, for exactly the same reasons.
This same topic is usually discussed using much less extreme examples, and is described as the problem of gun "cant". It can be easily seen that a little of what we were discussing above will cause the same type of error, but to a much less radical extent. No degree of gun cant is good.
To get really radical, what happens if the gun is fired while upside down? Using the same sights, of course, and everything else the same. Now it's easy to figure out. The compensation for drop which was throwing the ball to right or left in the examples above will now throw the ball down. No compensation for drop is present here, either, though, so full drop will occur. The barrel throws the ball 9.75 inches low and gravity pulls it another 9.75 inches low, so it will hit the target directly below the 'X', but 19.5 inches low.
Mental gymnastics, done before we even approach the shooting site, can teach us a lot about how our guns work. Knowing the intimate details of the factors involved in such problems as are discussed here can in no way be a bad thing, if our aim is to become better, smarter shooters.
Copyright © B. E. Spencer 2000 All rights reserved.