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PostPosted: Wed Jul 31, 2019 6:28 pm 
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Can the gears in the scope turrets go wonky? So you can sight it in fine at 25 yards but not at 100?

I screwed up the last time I was out and didn't know where I ended up sighting in my gill gun, so today I went out intending to sight it in on the 25 yd range and then find the number of clicks needed to sight it in at 100.

At 25 yards it shot 7" high, so I must have left it set somewhere near the 100 yard setting. I clicked it down 28 clicks (in 3 or 4 steps) til it was spot on at 25 yards.

Took it to the 100 yard range without changing anything, figuring it (hopefully) would shoot about 7" low. Close. It was 8" low, just at the very bottom of the target paper. I figured I need to raise it about 28-32 clicks to raise it 7-8" (assuming 1/4" per click).

Then the fun began. Clicked it up 8 clicks figuring it SHOULD come up about 2". I could not find those holes (I may need a better spotting scope). Adjusted it another 12 clicks, hoping to hit 3" higher, for a total of 5" higher, which would still be 3" low. Again I couldn't find those holes. Did another 12 clicks for a total of 32, which should have raised it 8" and put it right on. Could not find those holes either. So I gave up in disgust and fired of my last 10 rounds. Could not find them either.

But when I went up to retrieve my target I did find 3 holes close to the bulls-eye (so yes I do need a better spotting scope :-( ) ... and 6 holes in the backing paper just about 8" high. But also NO holes around the 6" and 3" low spots.

Could some of the gear teeth have been stripped so when I thought I was raising it 8 clicks/2" it just "wandered off the paper", and didn't come back until I got the full 32 clicks in (that would be one complete turn and almost 1/2 of a second turn)?

Is there something inside the scope that could come loose, and then correct itself at a different setting? I did check that the scope and mount were secure on the gun and the barrel was snug to the stock.

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PostPosted: Wed Jul 31, 2019 11:20 pm 
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You did not say what kind of scope you have. Is it a variable power scope?

It is better to shoot it in at 100 yards first.
I don't think they have gears, but a three point positioning system with one point a spring loaded stationary point. The other two points are 90 degrees of each other. The cross hair erector/tube will sometimes stick and not move smoothly depending on how well it was finished and might need a little tap to make it respond to the change in the xy coordinates. Some systems might have two springs loaded points or perhaps a curved flat spring to keep the tube pressed against the adjustment screws. Also if the adjustment screw threads are not machined to close enough tolerances there is room for wobble.


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PostPosted: Thu Aug 01, 2019 4:39 pm 
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Thanks oldernavy.

I don't know what the scope is. There are no markings on it; just an old straight tube scope for a .22. Just the smallest flare for the adjustable eyepiece. It has the teardrop shaped covers for the turrets. The mount is bolted to the side of the barrel and bent over to put the scope over the barrel (high enough that I can still look under it and use the iron sights). It is variable a bit, but with no markings I don't know how much. But I can fiddle with the eyepiece until I can see the bullseye at 100 yards. Just not the holes in the paper. I use the spotting scope for that.

That is just a cheap (-ish) Canadian Tire scope, a Celestron Cavalry 25X to 75X. And looking at it now I see it was set at 50X. I could see the first three shots at the bottom of the paper but not the three which were near the bullseye. I may not have even looked high enough to see the 6 just above the target, on the backing paper. But maybe next time I'll try the 75X setting.

But that won't let me see holes that aren't there. I.E. the ones that should have been near 5" and 3" low. I wonder if the three near the bull were actually from the first 8 clicks? They SHOULD have raised it 2", not 8". Then the six just off the target at the top would be the next 12 clicks, and my final 12 clicks were completely off the target, the backing paper and the target stand, somewhere up into the berm?

Most scopes have printed right on them 1 click = 1/4" at 100 yards. This one has no legend printed but 1 click at 25 yards gave 1/4".

:thnk: Damn! I think I see what happened! The scope manufacturers can't know what ammo you are using so the "one click" raises the POINT OF AIM 1/4" ... in a straight line with no consideration for a ballistic curve. Some how I had it in my mind that it was raising the point of impact. Dumb, dumb, dumb! If I clamp the rifle in one position I can 'click' until I'm blue in the face, and move the crosshairs all over the target but never change the point of impact. So if each click is 1/4" at 25 yards it will be 4X that at 100 yards, i.e. 1"! So those 3 near the bull were most likely from the first 8 clicks.

If I'm right then I just saved me taking a perfectly good scope apart to try to fix it. Next time out I'll take back those 2 sets of 12 clicks and start on the 100. And still use a big sheet of backing paper. AND try the spotting scope at 75X. Hopefully it should be pretty close to right on. (Then I'll have to find some way of marking the 100 and the 25 yard settings right on the turret. Or at least a pair of dots aligned on the wheel and on the frame to be a nominal zero.)

Funny, did ask some other members at the range "I'm getting 1/4" per click at 25 yards. Most scopes are set to give 1/4" at 100. Now that it is sighted in at 25, how many clicks should I start with to sight it in at 100?" One guy said he is only concerned about shooting groups so he doesn't care about the bullseye. The other said he mostly shoots iron sights. Neither one caught my confusion between point of aim and point of impact.

The rifle is an old Stevens 87B. It may be older than I am but it still shoots better than I can :D

oldernavy wrote:
You did not say what kind of scope you have. Is it a variable power scope?

It is better to shoot it in at 100 yards first.
I don't think they have gears, but a three point positioning system with one point a spring loaded stationary point. The other two points are 90 degrees of each other. The cross hair erector/tube will sometimes stick and not move smoothly depending on how well it was finished and might need a little tap to make it respond to the change in the xy coordinates. Some systems might have two springs loaded points or perhaps a curved flat spring to keep the tube pressed against the adjustment screws. Also if the adjustment screw threads are not machined to close enough tolerances there is room for wobble.

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PostPosted: Thu Aug 01, 2019 5:33 pm 
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Kimfella,

That is why I said sight it in at 100 yards. Most scopes change point of impact 1/4 MOA per click at 100 yards or also stated 1/4" at 100 yards. Target scopes are often 1/8 moa at 100 yards.

The reason for a finer division on a target scope is when you start going 400 yards 1/4 click on a less expensive scope is one inch. It is only 1/2 inch at 400 yards with a 1/8 moa per click.

I am happy you considered bullet drop compensation as I normally do sight in at one hundred yards and then start calculating bullet drop at 200-400 yards using a ballistic calculator vs ballistic coefficient starting with a horizontal shot. Of course a 22 rim fire is a different matter since you normally don't shoot much beyond 100 yards.

So you reasoned it out yourself. Great.


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