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Rifles, Reloading, Optics, Equipment
Long Range Scopes and Other Optics
Good Scope Mounting Tools
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<blockquote data-quote="bruce_ventura" data-source="post: 848537" data-attributes="member: 34084"><p>Gunner, I suppose this would happen if you located the anti-cant indicator on the rail, instead of the scope tube as Woods' photo shows. Attaching the anti-cant indicator to the scope tube is the preferred method for the following reasons.</p><p></p><p>First, I've calculated the alignment errors and the resulting scope cant sensitivity many times, and I've confirmed those calculations in live fire experiments. Here is what I've found. If the scope cant is removed but the scope vertical turret axis is not aligned to pass through the bore, the scope optical axis and the bore axis will not be parallel in the horizontal plane. The two axes will cross at the zero range. There will be a small but measurable horizontal offset in the point of aim that increases with distance from the zero range. </p><p></p><p>Horizontal offsets in the scope tube relative to the rifle bore can occur due for a variety of reasons. The most common source is left-right asymmetry in the ring attachment to the base, but base installation, height of the rings and barrel alignment to the receiver can also be factors. </p><p></p><p>Using the EXD and <strong><em>HighPower</em></strong>Optics Reticle Alignment Tools essentially eliminates this source of aiming error, but can result in the scope tube being canted when the rail is level (not canted). In this case, attaching the anti-cant indicator to the rail will cause a scope cant. </p><p></p><p>Second, if the anti-cant indicator is attached to the scope tube the indicator can easily be rotated the required number of degrees (for that bullet and MV) to compensate for spin drift. Doing the same thing with the anti-cant indicator attached to the rail is more difficult because it requires the scope to be precisely rotated in the rings. </p><p></p><p>None of these alignment criteria are sensitive 2-10 MOA of alignment error. Holding scope cant error to within 30 MOA (0.5 degree) is usually adequate for long-range shooting. I say "usually", because it really depends on the caliber (the point of aim error due to scope cant is proportional to time of flight). I assume we're talking about flat-shooting calibers with adequate bullet energy at long range. </p><p></p><p>Also, the spirit levels found in scope alignment tools and anti-cant indicators have a sensitivity of 30 MOA per mm of bubble travel. In my experience, an error of 10 MOA cannot be measured reliably without a machinists level, and no one I know uses a machinists level in the field to remove scope cant.</p></blockquote><p></p>
[QUOTE="bruce_ventura, post: 848537, member: 34084"] Gunner, I suppose this would happen if you located the anti-cant indicator on the rail, instead of the scope tube as Woods' photo shows. Attaching the anti-cant indicator to the scope tube is the preferred method for the following reasons. First, I've calculated the alignment errors and the resulting scope cant sensitivity many times, and I've confirmed those calculations in live fire experiments. Here is what I've found. If the scope cant is removed but the scope vertical turret axis is not aligned to pass through the bore, the scope optical axis and the bore axis will not be parallel in the horizontal plane. The two axes will cross at the zero range. There will be a small but measurable horizontal offset in the point of aim that increases with distance from the zero range. Horizontal offsets in the scope tube relative to the rifle bore can occur due for a variety of reasons. The most common source is left-right asymmetry in the ring attachment to the base, but base installation, height of the rings and barrel alignment to the receiver can also be factors. Using the EXD and [B][I]HighPower[/I][/B]Optics Reticle Alignment Tools essentially eliminates this source of aiming error, but can result in the scope tube being canted when the rail is level (not canted). In this case, attaching the anti-cant indicator to the rail will cause a scope cant. Second, if the anti-cant indicator is attached to the scope tube the indicator can easily be rotated the required number of degrees (for that bullet and MV) to compensate for spin drift. Doing the same thing with the anti-cant indicator attached to the rail is more difficult because it requires the scope to be precisely rotated in the rings. None of these alignment criteria are sensitive 2-10 MOA of alignment error. Holding scope cant error to within 30 MOA (0.5 degree) is usually adequate for long-range shooting. I say "usually", because it really depends on the caliber (the point of aim error due to scope cant is proportional to time of flight). I assume we're talking about flat-shooting calibers with adequate bullet energy at long range. Also, the spirit levels found in scope alignment tools and anti-cant indicators have a sensitivity of 30 MOA per mm of bubble travel. In my experience, an error of 10 MOA cannot be measured reliably without a machinists level, and no one I know uses a machinists level in the field to remove scope cant. [/QUOTE]
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