Pneumatic Pnews

Monday, August 21, 2006

Pellet trajectory - what you need to know

by Tex Force

Some people think a pellet shoots straight for a while and then starts to drop slowly from the bore line. Others believe that a pellet actually rises after it leaves the muzzle, then peaks at some distance from the gun before starting to drop. Neither of these impressions is correct. A pellet begins to drop the instant it leaves the muzzle. Let's see what actually happens.

The rise is an optical illusion
Sometimes, when the sun is at my back, I have actually seen a pellet seem to rise above the aim point of my crosshairs, then sink again at some point far away from the gun. This is an optical illusion caused by pointing the telescopic sight downward through the trajectory of the pellet. You do that when sighting in to compensate for the drop of the pellet. But, when the scope is looking straight ahead, the bore is actually tilted upward and the pellet seems to rise.


The pellet doesn't rise, but angling the scope down makes it appear that way.


Pellets and bullets do not generate lift
Actually, they do generate a tiny bit of lift, but it's so small that it doesn't make them rise. What it does is retard their fall by an infinitesimal amount. The amount is too small to measure. For all practical purposes, pellets and bullets generate no lift. They fall the instant they are not supported by the barrel. If you were to shoot a pellet from a barrel that's level with the ground and were also to drop a pellet at the same instant, both would stroke the ground at the same time. The pellet you dropped would land beside you and the pellet you shot would land far away. How far, you ask? Maybe 100 yards or so.

Diabolo pellets have extreme drag
Every watch a badminton birdie (shuttlecock)? It leaves the racket at high velocity, but slows to almost nothing by the time it has gone 30 feet. That's because its skirt creates a huge amount of drag. This air-braking slows the birdie faster than anyone can imagine. Well, a wasp-waisted, hollow-tailed pellet (called a diabolo) does much the same thing. It doesn't create as much drag as a badminton birdie, plus it is both smaller and denser, being made of lead, so a pellet may go as far as 500 yards when the muzzle is elevated to shoot as far as possible. But, a solid bullet (or pellet) of the same weight leaving the muzzle at the same velocity would go about three times farther.

The drop is not uniform
Over time, the fired pellet drops to earth at a uniform rate. Over distance, however, it doesn't. In the beginning the drop is small, but as the pellet slows (and we have just learned that it slows rapidly), the amount of drop seems to become more pronounced. What is happening is that the pellet is moving less far forward with every yard it advances, so the drop starts looking more pronounced. If you see it through a scope, it's very striking.

Each pellet type is different
Heavier pellets have less drag, and as will pellets with less of a wasp waist or a shallower skirt. However, the tradeoff is that such pellets are less accurate, as well. Always put accuracy at the top of your list and velocity at the bottom, because velocity means nothing until it connects with a target.

What can this knowledge of pellet performance do for you? We'll find out next time when we see how to best sight in a pellet rifle.

5 Comments:

  • I'm confused. Are the barrels canted upward in order create an arc? It would seem that if the pellet was ejected without upward cant the pellet would decline only and there would be not two point convergence for the scope.

    By Blogger D.B., at 8:20 PM, August 26, 2006  

  • D.B.,

    No, the goal is to mount the barrel straight (with the receiver), but that almost never happens. Usually the barrels point down a little.

    B.B.

    By Anonymous B.B. Pelletier, at 5:04 PM, August 27, 2006  

  • A pellet does really rise as the scope is adjusted. Your diagram assumes that the barrel is level. As the scope is adjusted, level sighting through the scope produces an actual trajectory similar to your second diagram, as the barrel is tilted upward.

    By Anonymous Anonymous, at 12:13 AM, October 08, 2006  

  • No, the pellet never rises. It can be made to appear to rise, relative to the crosshairs, but the pellet is always falling from the moment it leaves the muzzle.

    Even if you elevate the muzzle of the gun to point upward, the pellet falls away from a straight line projected from the muzzle to infinity as it travels downrange.

    The barrel isn't tilted upwards when you sight in. The scope is tilted downwards. So when you shoot at a distant target, you must elevate the scope, which in turn elevates the barrel. This is not a matter of mincing words, it is a fundamental difference.

    Tex

    By Anonymous Tex Force, at 8:18 AM, October 08, 2006  

  • I agree that there is a constant downward acceleration of gravity of 32.2 ft/sec/sec from the time the projectile leaves the barrel. As you adjust a rear sightupward, then level the view through the sights, the barrel is tilted upward.
    Neglecting drag and gravity, after firing with a tilted barrel, there would be a straight rising trajectory of constant velocity, which could be resolved into horizontal and vertical constant velocity components. There is a vertical upward velocity component, which is what one would call rise. Including gravity, there is a constant downward acceleration, acceleration being a rate of change of velocity. At the instant of firing, the downward velocity is zero, it increases to 32 ft/sec in the first second, 64 ft per second in the second second, 96 ft/sec in the 3rd second, etc. Until the downward velocity component due to gravity is greater than the upward velocity component of the initial trajectory, the projectile is rising.
    The fact that the gravity downward velocity is increasing at a constant rate of change is the main reason the trajectory appears to fall much faster further out, because it is falling at a much faster rate with time, regardless of drag. It is true that drag is a negative acceleration acting in the line of the trajectory and does contribute to the described observation, since it is acting in the opposite direction of the trajectory, its effect is usually more in the horzontal direction. The main reason for the higher velocity of drop is due to increased velocity downward due to gravitational acceleration.
    Thanks, Jon

    By Anonymous Anonymous, at 5:54 AM, October 09, 2006  

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