Pneumatic Pnews

Monday, August 28, 2006

How best to sight-in an air rifle

by Tex Force

In our last blog, we saw what the trajectory of a pellet looks like and how scopes are adjusted to compensate for it. Today, we'll learn the best way to use this information.

One zero point or two?
It's your choice when you sight in a scope whether you get just one zero point or two. If you look at the pellet's trajectory again, you'll see that to get one zero point, you must angle (sight-in) the scope so it just grazes the top of the falling pellet at the one distance you desire. Let's say you want to zero your scope for ten yards. All you do is adjust the scope so that the position of the pellet coincides with the intersection of the crosshairs at 10 yards. At all other distances, the shot will be below the intersection of the crosshairs.

New shooters often think this is exactly what they want, because it seems so simple and straightforward. But this is not how to sight-in a scope. They either haven't thought it through, or they don't know what's going on with the pellet in flight. But, you do, because you read last week's blog (Pellet trajectory - what you need to know). You know there is a much better way to sight-in a scope.


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


Two zero points
If you look at the pellet's curving trajectory and the scope's straight line of sight, you soon realize that, if the scope is adjusted correctly, it will cross the pellet's path at a near point. Then, the pellet will seem to rise above the intersection of the crosshairs for several yards before it falls back to the intersection again. The pellet is falling all the time, of course, but the way the scope is adjusted in this second version, it looks down through the trajectory and makes this illusion of a rising pellet.

What you also learned in last week's blog is that pellets slow down rapidly. So, the curve of a pellet's trajectory becomes rapidly more pronounced the farther it gets from the muzzle. Initial velocity and the pellet's own ballistic characteristics determine what the curve looks like, but there is a common distance at which it makes good sense to sight in your rifle.

The magic!
The magic first sight-in point for adult pellet guns with velocities in the 800 to 900 f.p.s. range is 20 yards. Let's look at one specific case. A pellet rifle with a muzzle velocity of 850 f.p.s. is sighted to strike the intersection of the crosshairs at 20 yards. The pellet will then "rise" about one pellet's width above the aim point. Where exactly that will be I'm not sure, but it's about 28-29 yards. The pellet then starts sinking again and crosses the intersection of the crosshairs at 33 to 35 yards. By 40 yards it will be one pellet's width below the intersection of the crosshairs.

What this gives you is a rifle that is essentially zeroed from 20 to 40 yards. How much better is that than a gun that can hit the aimpoint at only 10 yards? There is a drawback. The way scope mounts and scopes work, you can't adjust these numbers very much at all. You can't, for example, have a rifle that's sighted in between 10 and 30 yards. The trajectory simply won't tolerate it. It's got to be 20 yards for the near point and anywhere from 30 to 40 yards, depending on the muzzle velocity, for the far point. Throw a tantrum if you must, but the numbers I've given you are THE numbers.

Where this came from
Don't blame me. I'm just telling you the facts. This phenomenon was discovered in the 1990s by American field target shooters who set up their scopes for adjustment for every shot. They noticed there was a band of distances at which they didn't have to raise or lower their sights. When I shot a Daystate Harrier in competition, my rifle liked 20 to 30 yards. The pellet was virtually on target at any distance between those numbers. Because I adjusted the reticle whenever it was needed, it didn't matter one iota where the flat spot occurred. Everyone else had the same sight-in data as I had.

Notice, I did not mention caliber in all of this? That's because caliber doesn't matter. My .177 Harrier, shooting 10.5-grain Crosman Premiers at 860 f.p.s., had virtually the same trajectory as someone else's .20 caliber Career 707 shooting the same speed.

3 Comments:

  • What are the effects of scope height? Pieces to elevate the mounting rails are popular among target shooters but what are the major drawbacks?

    Markus

    By Anonymous Anonymous, at 4:02 AM, August 29, 2006  

  • What are the sight in distances for gun within the 500 to 700 range? If you use a "Bug Buster" scope set at say 15feet is there only one point of intersection?

    By Anonymous Anonymous, at 7:27 AM, August 30, 2006  

  • The near distance WILL ALWAYS be 20 yards. If you sight-in closer than that, your scope will have a very limited span of distance at which it stays sighted-in. For an airgun shooting 500 f.p.s., the far distance will be around 25-26 yards.

    Tex

    By Anonymous Tex Force, at 8:04 AM, August 30, 2006  

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