Another cause of scope shift: over-adjusted scope knobs

By B.B. Pelletier

In my March 14 report, What causes scope shift?, I promised to return to some of the other causes of scope shift. One more way to acquire scope shift is when either one of your scope knobs is adjusted out too far, allowing the reticle to flop around.

How your knobs got over-adjusted
Inside the scope tube, the reticle is housed in a smaller tube called the erector tube, which rests on springs that are on the opposite side of the windage and elevation knobs. When you adjust the elevation knob “down,” you are actually applying pressure to one side of the erector tube, which compresses the spring on the opposite side. If you adjust either knob as far as it will go, the spring either becomes bound up and refuses to move, or it becomes so relaxed that the adjustment knob starts feeling mushy and indistinct.

When the adjustments feel mushy, the spring that pushes against the erector tube is relaxed and not able to keep proper tension on the tube. A bump to the rifle or even regular recoil can push the erector tube to a different spot – and you end up with scope shift! Some scopes are designed to minimize this problem – but yours may not be, so keep scope knobs fairly well centered (up/down and left/right).

Get more out of your scope without risking scope shift
How do you get on target without using the scope’s adjustment knobs? Use an adjustable scope mount! It lets you zero a scope without using its internal adjustments. When you want to make small changes, you’ll have the scope’s entire adjustability available.

Adjustable scope mounts are more difficult to set up initially. Once they’ve been properly adjusted, the scope is far easier to use – you put in time up front to save time later. If you don’t want to remount all your scopes, follow my rule of thumb: use adjustable mounts on your most precise airguns and fixed mounts on guns used for more general shooting.

5 thoughts on “Another cause of scope shift: over-adjusted scope knobs

  1. BB,

    I recently purchased a Gamo 1000 (my very first airgun) and have been using your blog extensivly for research purposes. I mounted a BSA 2-7X32MM using non-adjustable weaver rings but was not able to get enough adjustment out of the scope. After some shimming and whatnot I was able to get some degree of accuracy (1 inch groups at 20 yards with the occasional stray shot. I think this has to do with the copperhead match pellets I am using). I do, however, have the elevation knob adjusted all the way down. So much so that when I turn it there is a lull with no clicks followed by some clicks with the pattern repeating. I also put my gun in a vice to see if the adjustment knobs were working. While I could easily tell that the windage knob was actually moving the recitle I could not tell if the elevation knob was working or not. Are the degree changes in elevation smaller than windage? Do you suggest I get an adjustable mount or keep shimming? Also, does this clicking followed by a lull mean I might have inadvertantly broken my scope totally?

    Thanks in advance.


  2. Gamo Owner,

    A scope’s adjustment is equal amounts in both directions. As you have no doubt read on this blog, you have the elevation return spring almost compressed, and that’s what is making your adjustments so different and varaible.

    Try adjusting the elevation knob in the opposite direction. If you can hear the scope clicking when you wind the elevation screw halfway out, the scope is probably okay. I think it is, but you are in a spring-bound condition where the scope does not like to be.

    I advise you to get a set of B-Square AA adjustable scope rings for your rifle. They will give you the mounting correction you need to keep the scope knobs in the middle of their operating range.

    B.B.


  3. Thanks for your quick response. I really appreciate all your help. I plan to order the adjustable rings today and try out some new pellets that will hopefully be more accurate.


  4. Top tip for target shooting… Place your gun on a stable surface, put your eye in the sweet spot move your head around a little. If you see the reticle shift up/down or left/right relative to the target then you need to tweak the AO/parallax ring. Keep doing this until there’s no movement.

    Once completed and assuming your shooting from the same spot you should find your accuracy has increased significantly.


  5. My RWS has the C-Mount. Using a vise, I put the scope in midrange adjustment, then use the crude adjustment in the mount to get most of the zero on the rifle.

    To do this, you must attach the rings to the scope first, then slight adjustments can be made in windage and a lot of elevation can be made. After getting on paper, within 1″ at 25 yards, then use the scope adjustments.

    The wrenches with the C-Mount are worthless. Get a 1/4 metric socket wrench set, shortest possible. Lube threads with grease and coat the friction surfaces with liquid electrical rosin flux, this locks the friction surfaces once tightened yet breaks free once tension is released. Regular powdered rosin, dissolved in alcohol, works fine as does a rosin bag dusting (bowling pro shop or baseball). Rosin pitch seriously increases friction. Also great on the claw mounts. Not for threads!!

    On my rifle, I have maximum tilt for elevation for a 25 yard zero. Rifle is dead on there and at 55 yards. Nearly PBR for squirrel shooting.


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