Spats McGee
Administrator
Thank you, and thanks to everyone who responded to my questions. Now it makes sense.
How do you level your scope?
- Thread starter ndking1126
- Start date
That can be done at home without shooting bullets.Well, the way I am understanding your question, I have to say; YES. ….
Be Safe !!!
Another useful thing to have is a setup for zeroing your scope adjustments before you set the scope up on the gun. You may be able to do this by counting clicks to find the middle of the ranges of elevation and windage, but with some scopes the adjustment to optical center that way may not be exact. Stoney Point used to sell molded plastic v-blocks for optically zeroing scope adjustments and anyone with a 3D printer could make you a set, but I have just used 1/4" round head Nylon screws set into scrap wood as shown in the old illustration below. A change I made since was to notch the vertical wood between the screws so I could set them a little further apart for more stable scope support and I hot-melt glued the screws in to keep the screwdriver slots parallel to the scope.
The idea with that fixture is to tweak the scope adjustment knobs until rotating the scope no longer moves the center of the crosshairs off whatever you are looking at through the scope. Once you have that optical zero, you set the scope up on the gun as I described in post 27 or by other means of your choosing to get the vertical crosshair on a line that passes through the bore center. Once you have done that and you know the height of the scope above the bore line and assuming your base and rings don't introduce any left or right direction error, you will be able to dial in a zero as good as any bore sighter can produce. In other words, a zero you have only to correct for barrel vibration and recoil moment effects on POI.
If you have a long-range scope base with elevation, don't forget to subtract that number from your elevation setting to determine what your initial zero settings should be.
For example, I find the optical rotation center for my scope as above. I mount it with a base and ring set that puts the center of the scope 1.5 inches above my bore line. I have ammunition that will fire the 155 grain Sierra 2156 Palma MK from the gun at 3000 fps. The base has +20 MOA of elevation built in for long range. I decide to find the 100-yard zero first. So I put my load specifics into the simplified trajectory calculator at the JBM ballistics site. On that software I will do two things that may seem a little odd:
Also, I
The result will show a drop of -2 inches or 1.9 MOA at 100 yards. Since my scope height is 1.5 inches, the actual drop will be another -1.5 inches below my scope line of sight. So, for adjustment purposes, the drop is -2 inches plus -1.5 inches or -3.5 inches. At 100 yards, an inch is close enough to an MOA so I can use +3.5 MOA as the expected correction. I subtract the scope base elevation from that number to get the adjustment correction. That is,
+3.5 MOA - 20 MOA = -16.5 MOA,
so turn I turn the elevation knob down -16.5 MOA to get my trial zero at 100 yards.
If you want to correct to mathematically exact MOA just because you can, multiply pi times the actual range in yards and then divide the result by 300 yards to get the conversion factor. Divide the scope height by that conversion factor (1.0472 at 100 yards). For 1.5 inches that gives 1.43 MOA. Add 1.43 MOA to the 1.9 MOA from the table to get 3.33 MOA adjustment correction. In my case, I would subtract 20 MOA from that number and get -16.67 MOA sight adjustment to produce my trial zero.
Note that the error from using inches as MOA instead of mathematically exact MOA is just 0.167 MOA in this example. In reality, barrel vibration and the action of recoil moments on my hold will introduce more error than that, and I will have to dial in their correction by actual firing and record it. So there is no point in being tighter than using inches as MOA at 100 yards, IMHO. If you have 1/8 MOA clicks or 1/10 MOA clicks on your scope, you can dial in closer to correcting that small error. But I have to say from experience with a 1/8 MOA mil-dot scope that the time it takes to count all those clicks and the increased likelihood of losing count of a large number of clicks is something I found made such fine adjustments more a nuisance than a help. The standard 1/4 MOA adjustment is already finer than most people can hold in most real conditions what with wind moving around and so on. Mid Tompkins told a class I took that he doesn't want sight adjustments finer than 1/2 MOA for long-range because the wind usually changes more than 1/4 MOA in the time it takes to add a click to the adjustment. He said our scores would be higher if we learned to correct errors that small by holding off slightly.
The idea with that fixture is to tweak the scope adjustment knobs until rotating the scope no longer moves the center of the crosshairs off whatever you are looking at through the scope. Once you have that optical zero, you set the scope up on the gun as I described in post 27 or by other means of your choosing to get the vertical crosshair on a line that passes through the bore center. Once you have done that and you know the height of the scope above the bore line and assuming your base and rings don't introduce any left or right direction error, you will be able to dial in a zero as good as any bore sighter can produce. In other words, a zero you have only to correct for barrel vibration and recoil moment effects on POI.
If you have a long-range scope base with elevation, don't forget to subtract that number from your elevation setting to determine what your initial zero settings should be.
For example, I find the optical rotation center for my scope as above. I mount it with a base and ring set that puts the center of the scope 1.5 inches above my bore line. I have ammunition that will fire the 155 grain Sierra 2156 Palma MK from the gun at 3000 fps. The base has +20 MOA of elevation built in for long range. I decide to find the 100-yard zero first. So I put my load specifics into the simplified trajectory calculator at the JBM ballistics site. On that software I will do two things that may seem a little odd:
- Enter the zero range as 1 yard.
- Enter the scope height as zero.
Also, I
- Set the range and range increment for the zero range I want.
- Also, I set wind and target speed to zero.
The result will show a drop of -2 inches or 1.9 MOA at 100 yards. Since my scope height is 1.5 inches, the actual drop will be another -1.5 inches below my scope line of sight. So, for adjustment purposes, the drop is -2 inches plus -1.5 inches or -3.5 inches. At 100 yards, an inch is close enough to an MOA so I can use +3.5 MOA as the expected correction. I subtract the scope base elevation from that number to get the adjustment correction. That is,
+3.5 MOA - 20 MOA = -16.5 MOA,
so turn I turn the elevation knob down -16.5 MOA to get my trial zero at 100 yards.
If you want to correct to mathematically exact MOA just because you can, multiply pi times the actual range in yards and then divide the result by 300 yards to get the conversion factor. Divide the scope height by that conversion factor (1.0472 at 100 yards). For 1.5 inches that gives 1.43 MOA. Add 1.43 MOA to the 1.9 MOA from the table to get 3.33 MOA adjustment correction. In my case, I would subtract 20 MOA from that number and get -16.67 MOA sight adjustment to produce my trial zero.
Note that the error from using inches as MOA instead of mathematically exact MOA is just 0.167 MOA in this example. In reality, barrel vibration and the action of recoil moments on my hold will introduce more error than that, and I will have to dial in their correction by actual firing and record it. So there is no point in being tighter than using inches as MOA at 100 yards, IMHO. If you have 1/8 MOA clicks or 1/10 MOA clicks on your scope, you can dial in closer to correcting that small error. But I have to say from experience with a 1/8 MOA mil-dot scope that the time it takes to count all those clicks and the increased likelihood of losing count of a large number of clicks is something I found made such fine adjustments more a nuisance than a help. The standard 1/4 MOA adjustment is already finer than most people can hold in most real conditions what with wind moving around and so on. Mid Tompkins told a class I took that he doesn't want sight adjustments finer than 1/2 MOA for long-range because the wind usually changes more than 1/4 MOA in the time it takes to add a click to the adjustment. He said our scores would be higher if we learned to correct errors that small by holding off slightly.
All the scopes I've put the erector tube lenses on scope optical (outer tube) center had more clicks up and right, towards the knobs, to their mechanical limits. And the reticle/LOS stopped moving several clicks before the limit. Proof to me the erector tube was against the outer tube or something else.
Nobody in the shooting sports used the trig MOA value until the hand held scientific calculators came out in the 1960's.
Metallic sights can be spaced to whatever value you want. The original standard for match aperture rear sight was 40 TPI lead screws, 12 clicks per turn, 30 inch sight radius. Externally adjusted target scope mounts had a 7.200 inch spacing, 40 TPI lead screws. .0005" per click.
Very few, if any, internally adjusted scopes are exact to published click values. Their lenses have a few percent tolerance in focal lengths.
Exact trig value below was calculated to 102 decimal places for those wanting more precision:
1.04719753642832854694747069666400334739860873986429
830552235157457471965151538005004775737357536725837... inches per 100 yards...
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