There are two ways to check for repeatable tracking. One is by sticking a collimator in the muzzle of the rifle, then turning the adjustment dials while watching the reticle move across the collimator screen. If a scopeís reticle doesnít track reliably across the screen, it wonít in the field either.
The other thing a collimator can discern is reticle shift throughout the magnification range of the scope. (This test is performed only on second focal-plane scopes, because there canít be any shift with first focal-plane scopes.) The Bushnell 6500 passed both collimator tests.
The click-test, however, is only preliminary, because some scopes pass but donít do as well when actually used at the range. This happened, slightly, with the 6500. The range test involved a version of ďshooting the square.Ē There are different versions of this game, but the one I use with varmint scopes that normally are mounted on very accurate rifles is to first shoot a group, then click 5 inches to the right and shoot one shot, then click 5 inches down and shoot one shot, 5 inches left and shoot one shot, and finally five inches up and shoot one shot. If the scope tracks correctly, the final shot should be back in the first group. If it isnít, then I shoot a few more times, to see if the recoil will jiggle the adjustments into place.
The 6500 was mounted on a heavy-barreled, laminated-stock Remington 700 chambered for the .223 Remington. I ďaccurizedĒ this rifle myself, and it turned out to be perhaps the most accurate factory varmint rifle Iíve ever fired. When the barrel was new it would normally shoot ľ" five-shot groups at 100 yards with carefully assembled loads featuring the 50-grain Nosler Ballistic Tip and 26.0 grains of Ramshot TAC. After several years of use in the prairie dog fields it wonít quite do that anymore, but is still accurate enough for scope testing.
In this instance the first three-shot group measured 0.38", pretty good for a rifle thatís slain a few thousand prairie dogs. The last shot after shooting the square did not land amid the shots from the initial group, but about an inch to the left. Two more shots landed on either side, confirming the point of impact.
This actually isnít that big a deal, because the elevation was right on, and the elevation adjustment proved to be highly repeatable when cranked up and down, both in the collimator test and at the range. Most of us primarily use the elevation adjustment anyway, holding off for windage.
The other aspect of this square test is seeing if the clicks match the claimed adjustment value, in this instance ľ" at 100 yards. This actually isnít that big of a deal either, as long as the adjustments are consistent, but it does make a difference when weíre calculating how much elevation to crank when shooting at different yardages. If the clicks arenít actually ľ", then we need to know that ó and in many scopes Iíve found they arenít exactly 0.25".
In this instance the elevation clicks were right on, but the windage clicks averaged 0.35". Again, no big deal, because the elevation clicks are what most of us use, most of the time. The clicks were firm and easily felt, The adjustment turrets feature dials that can be lifted up and then turned and clicked back down to set to zero, and thankfully arenít nearly as tall as some. The magnification ring and side-focus both worked smoothly and with no apparent slack.
Next came the optics test. This is a three-part test, the first test measuring the eye relief by shining a flashlight into the objective lens, then measuring how far back from the eyepiece the exit pupil focuses. Eye relief is advertised as 4.0", and measured 4.3" at 4.5x and 3.6" at 30x.
The second test is a subjective look through the scope at various magnifications, including near the sun or another bright light source. The 6500 was remarkably free of flare and chromatic aberration, and the field of view was flat and sharp at all but the very highest magnification. At 30x the view became a little degraded, lacking contrast even in bright sunlight, no doubt because even a 50mm objective lens provides only a 1.67mm exit pupil. At 24x it was fine, however. The lenses have Bushnellís Rainguard hydrophobic coating, but I didnít test it, having done so enough in the past to know that it works. Rainguard doesnít prevent all moisture from sticking to the lens, but does repel enough to provide an image useable for most shooting.
The last part of the optics test is done at night, on a chart of my own design. This has 10 alternating black and white lines that start 1" wide at the top of the chart, decreasing to 1/16" at the bottom. The chart is tacked to the back fence in my yard, then illuminated by the 100-watt bulb of my back porch light. The scope is aimed at the chart from an upstairs window of a dark room, so that the porch light doesnít introduce any stray light to the view.
The test is done at 25 yards, with the scope set on 6x. The short distance might seem strange, but at longer ranges the atmosphere itself starts to intrude, and weíre trying to test optics, not air. The scope is set on 6x because just about any modern variable has an objective lens large enough (at least 36mm) to create a 6mm exit pupil with the scope on 6x, and my eyeís pupil expands to about 6mm in very dim light. Once again, we are trying to test the optical quality of different scopes, and having them all on 6x with an eye-matching exit pupil provides a level playing field. The rifle (or just the scope, if it isnít on a rifle) is placed on a pillow on the windowsill, so that the scope can be peered through when itís absolutely still. This setup also is essential, because scope vibration can skew the test.
Scopes are rated by how many lines can be clearly seen on the chart. An inexpensive scope, for instance, might rate a 5 (the 3/8" line), while a top-notch scope might rate a 7 (the 3/16" line). Sometimes Iíll add a + sign to the rating number, indicating that the line could be seen very clearly. The chart is relatively easy to use, because all the lines below the bottom-most clear line will appear gray, like a zebra in the distance.
I call the rating the BC number, for Barsness Crude, since this obviously isnít as precise as using a million-dollar laboratory device to precisely measure the percentage of available light passing through the scope. It does, however, provide more information, because the alternating lines indicate something about a scopeís brightness and sharpness, and both affect how we see things. (Since the lines are black and white, however, they donít tell us anything about color rendition. This isnít as important, partly because how individual humans perceive color varies considerably.) The Bushnell 6500 rated a 6, which is darn good.
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