POI shift when changing the scope’s power
by Tom Gaylord
Writing as B.B. Pelletier
This report covers:
- The last test
- First group at 14X
- Second group at 6X
- Third group at 14 X
- The final group at 6X
This report is an unprecedented final look at how the point of impact (POI) changes when the power of a variable-power scope is changed. I linked to the 3 earlier reports in which this phenomenon was tested (it wasn’t tested in the first report of the Aeon scope, but I included it for continuity). The scope used in today’s report is different, so we will see whether that makes a difference to the results.
I had no intention of conducting this test, but then reader Silver Eagle asked this:
Can we try this same test with a airgun that does not have the variable pressure that a PCP has? An accurate springer such as a TX200 or similar would help narrow it down if it is the scope or the rifle.
The last test
I had thought about that when I conducted the last test and noticed that my groups were larger and smaller depending on the air pressure in the gun when I started shooting. I mentioned that in the report, and Silver Eagle caught it. So what he asked was the same thing I also wondered — if there was no change that resulted from the gun, what would the groups do when the scope’s magnification was changed? Granted I’m using a different variable scope for today’s test, but if this phenomenon is common to all variables, we should still see it.
This will be the last time I conduct this test. It’s not that I have proven or disproven anything conclusively. In fact, just the opposite. I have opened a can of worms and they are scattering everywhere! There are so many variables here that I don’t have enough time left on earth to test them all. But I wanted to see what kind of results I got from a different kind of airgun and a different scope under similar test conditions.
Silver Eagle suggested the test. Why not shoot my TX200 Mark III at 50 yards and see whether changing the power on its variable scope changes the POI? The rifle is currently scoped with the 4.5-14X42 Sidewinder Tactical scope that I have found to be one of the finest riflescopes I have ever used. While it doesn’t have the magnification of the Aeon 8-32 AO scope with trajectory reticle that I tested before, this scope is astonishingly clear. It should be able to perform this test at 50 yards on much lower power.
I chose to shoot at 6 power and at 14 power. That should be enough separation to show a POI change if there is going to be one. For my pellet I selected the Crosman Premier 7.9-grain dome because this rifle likes it a lot. When I first shot the rifle on this day, the impact point of this pellet was about 2 inches low. An adjustment raised the POI into the bullseye. The rifle was fired with the stock rested directly on a sandbag, as that has proven to be the most accurate way to test this airgun.
First group at 14X
The first official group was fired with the scope set at 14 power. The wind was dead calm throughout the test, so conditions were ideal. On this first scope setting, 10 pellets went into 1.519-inches at 50 yards. The group is centered on the bull and is fairly round. This will be our baseline.
Second group at 6X
Next I adjusted the scope back to 6 power. The parallax appeared perfect, but at 6 power it’s impossible to know for certain at 50 yards. I shot another 10-shot group. These pellets went into 1.073-inches. The group is still well-centered, but the center of the group did drop by about 1/8-inch. That’s definite movement, but not enough to be concerned about.
Ten 7.9-grain Crosman Premier pellets landed in 1.073-inches at 50 yards when the scope was set on 6 power. The center of this group is in the same place left and right, but about 1/8-inch lower than the last group.
We learn two important things from this second group. First, there is a slight but definite shift in the center of the group. It’s not large, but it’s there. That addresses the question of whether the POI shifts when the magnification is changed. We have now seen it on all three of my tests with a pellet rifle at 50 yards, plus I told you about a much larger shift at 200 yards that my buddy Otho experienced.
The second thing we see is that the rifle grouped better with the scope set on lower power than when it was set on high. If you recall, I did a test of that phenomenon for you in response to the question posed by our Moscow correspondent, duskwight. Duskwight also showed that in his guest blog that was in response to the test I conducted a week earlier.
I don’t think the scope’s magnification has much to do with the group size. What I think is that as long as the image in the scope is clear you can do just as well with it set on low power as when it is set on high. And that’s exactly what I did next.
Third group at 14X
For the third group I adjusted the scope back to 14X. That should tell us two things. First, does the group shift back to where it was initially with the scope set at 14X, and second, is this group larger or smaller than the one shot on 6X?
This time 10 pellets went into a group measuring 0.916-inches between centers at 50 yards. So this group is a little smaller than the last group that was shot at 6X. But notice that the POI shifted back up again. Perhaps it isn’t in exactly the same place as the center of the first group, but it’s easy to see that it is higher than the group shot at 6X.
The final group at 6X
Now I adjusted the scope back to 6X to shoot the final group. Will the center of the group drop back down again?
This time 10 pellets went into 1.295-inches. And the center of the group definitely dropped below the enter of the last group. It’s even a little below that last group shot on 6X.
There are several conclusions we can draw from today’s results. First, there seems to be no question that the POI does shift when a variable power scope’s power is changed. I believe this differs from scope to scope. Some scopes will shift more than others. I recommend that you test your variable scopes this way on every airgun you own and see what results you get.
Next, I think this test demonstrates once more that the power of the scope has very little to do with how accurately you can shoot with it. While I was conducting this test I was also testing a Remington 788 30-30 rifle at 200 yards. I have a Leupold M8 4-power scope on that rifle, yet I was able to see the crosshairs against the black bullseye clearly at that distance. Optical clarity is the most important thing in a scope, I think.
Finally I was surprised to see my TX200 Mark III do better at 50 yards than my Talon SS. That Talon SS has long been my go-to air rifle when the ultimate in accuracy is required. This test taught me that the TX is another solid performer. I’m not saying the TX is more accurate than the Talon SS, because I don’t think it is. But for today’s test it was the ideal gun to use.
As I said at the start of this report, this is the last test I will do specifically on this question. Of course I will remember these results in all my future shooting, as I hope you will. This lesson is too hard-won to forget. Test your scopes under the conditions you intend using them, and remember their results. That should make you a more confident shooter, and confidence builds skill.