Scopes - Part 3
How high should a scope be?
by B.B. Pelletier
The first two sections of this report were titled poorly for the way I blog, so I went back and changed their titles to Scopes, Parts 1 and 2. It's too difficult to track anything else. You may have forgotten, but I'm using this section to write the outline of a book on airgun scope mounting.
Today, I want to look at scope height. How high do you want your scope to be? There are two considerations for scope height. The first is adjusting the height of the exit pupil (the light that comes out of the rear of the scope that lets you see the target and reticle) to the height of your eye and the second is the clearance of the scope above the parts of the gun. Let's discuss both.
Fit to the eye
Ideally, your rifle should mount to your shoulder with the eye coming into perfect alignment with the exit pupil. I say ideally because it seldom happens. You want to get as close to the ideal as you can so that parallax from different eye placement is reduced as much as possible. With some rifles this will demand a medium-height mount and with others a high mount. With a few, such as the AirForce rifles, which have an absolutely straight stock line, they demand ultra-high mounts (which are already built in to the gun).
Usually, the height of the cheekpiece is a clue to how high the scope will have to be, but a stock with a straight line (i.e., not much drop from the pistol grip to the butt) will fool you into thinking you can get by with a lower mount than really needed. You may need to live with a rifle for a period before you know what it needs scope-wise.
Fit to the gun
Obviously, the scope mount has to be high enough that the scope caps can be tightened around the scope tube. Scopes that have large objective lenses will need higher mounts to clear the gun, and guns that have level actions (most spring-piston air rifles) will need higher mounts for the scope to clear the gun. PCPs often have receivers that extend above the level of the barrel, thus providing some of this clearance.
However, scope length also comes into play here. If the scope is short enough, it won't get in the way of loading, regardless of how close it is to the spring tube. As long as it clears the tube, you're home free.
Some guns have special clearance needs. PCP repeaters with circular clips need higher mounts to clear the clip. Sidelever and underlever rifles, such as the RWS Diana 460, with tight breech access often need scopes higher for pellet loading.
A circular clip means the scope has to be mounted high in order to clear. Not this high. though. This was extra high because of long-range testing. I needed an adjustable scope mount and this B-Square was all I had on hand.
Disadvantages to mounting high
The higher you mount the scope the greater the effect of cant. Because the sight is so far from the bore, any angular difference will be magnified. Some shooters also believe that a lower scope helps flatten the trajectory. It does, but I don't find the difference to be so great that it concerns me. I first get the scope to rise to my eye and to clear the gun before I worry about flattening the trajectory. That's just personal; you have to decide what's important for your kind of shooting.