Shifting point of impact

[Sam]

http://www.hunt101.com/data/500/medium/target5.jpg

The target above was shot at 200 yds. from the bench on a sunny afternoon. The rifle was a Remington 700 SPS, 20" barrel and resting on a bipod, the stock was supported.

The scope was a Nikon Buckmaster 6x18. I've had the scope for about 3 years and it's only been mounted on this rifle. The mount is not loose, I checked.

The point of aim was the same, the center of the bullseye.

The first three shots were made using 10X magnification. All went high.

The remaining shots were made using 18X magnification (maximum). Please ignore the writing on the target, my mistake. As you can see, the shots at 18X were closer than the shots made at 10X.

Can anyone explain the shifting point of impact? I'm not that knowledgeable about scopes/optics and such. Educate me please. Or is my scope broken?

[matemike]

At 200 yards, even with 18X, I´d like to see a different colored 10 circle. In this case, a white bullseye instead of the whole darn circle being black.
JMO...it might help...and inch lines for measuring.

But:
Does that scope have a paralax adjustment?
What was the wind doing that day? any increase, decrease, or direction change between magnifiaction adjustments?

[a0cake]

Could be a couple things. Issues like this are often hard to pin down.

Equipment wise, all Second Focal Plane scopes will experience some kind of POI shift along the magnification spectrum, at least theoretically. On the higher end SFP scopes such as Schmidt and Bender, you will not see any kind of POI shift in actual use.

With cheaper optics, the POI shift with magnification change can manifest itself noticeably. The good news is that it's repeatable. I suggest shooting multiple 5 shot groups at 18x and 10x. If the POI shift is consistent and repeatable, you've narrowed it down to the optic.

Now, shooter wise...Are you "loading the bipod" for each group? If you shift your weight forward and lean into the bipod as you should for one group, but then back off for the next group, you'll likely see a vertical shift in POI. Some call this "bipod hop." A proper consistent bipod load will eliminate this phenomenon.

Of course you now have your other main culprits for POI shift such as inconsistent cheek weld, parallax, etc.

Have you also checked to make sure your stock is not coming loose?

Sometimes a combination of shooter error and random issues can conspire against you and cause you to make determinations that are actually not related to the root of the problem...IE...it's the scope.

If I had to place a bet, I'd put it on the above or the scope shifting POI with magnification change.

The best way to diagnose the issue, as I said earlier, is to shoot 5-10 groups of 5 on each magnification setting (10x and 18x) while really concentrating on your fundamentals. I think the results will offer better insight than anybody else can without being there.

[Failure2Stop]

Now, shooter wise...Are you "loading the bipod" for each group? If you shift your weight forward and lean into the bipod as you should for one group, but then back off for the next group, you'll likely see a vertical shift in POI. Some call this "bipod hop." A proper consistent bipod load will eliminate this phenomenon.


My vote, barring optic issues.