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Rifles, Reloading, Optics, Equipment
Long Range Scopes and Other Optics
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<blockquote data-quote="bruce_ventura" data-source="post: 1045868" data-attributes="member: 34084"><p>I couldn't agree more. Aside from wind and range estimation, there are four ballistic errors that can creep into long range shots: </p><p>1. Canting error</p><p>2. Spin drift </p><p>3. Reticle misalignment to the bore</p><p>4. Coriolis effect</p><p></p><p>If we focus exclusively on the big one (canting error) and ignore the others because we assume they are small, we could get lucky and they could cancel each other out to some extent, or we could get unlucky and they could combine with each other to create a significant aiming error. <strong>The goal of long range shooting is to minimize them all, so that we're not relying on luck.</strong> </p><p></p><p>Coriolis is the only error that can't be eliminated ahead of time because we have to know where we are on the globe and what direction we're shooting. The rest of these errors are relatively easy to eliminate, <u>so why not just zero them out and be done with it?</u> </p><p>1. Canting error: eliminating canting error requires the use of a properly aligned anti-cant indicator.</p><p>2. Spin drift: eliminating spin drift error requires the use of a ballistic computer and knowledge of the bullet length and rifling twist rate. </p><p>3. Reticle misalignment to the bore: eliminating this error is easiest to do with a reticle alignment tool like the RingTrue tool from HighPowerOptics. Most other methods reduce but not necessarily eliminate the error. How small does this error need to be?</p><p></p><p>In practice, I think the goal is to get the reticle and/or elevation turret axis aligned to the rifle bore within 0.125", which is less than half of a ¼ MOA click at long range when the rifle is zeroed at 100 yds. That angular error would be too small to contribute to a significant aiming error regardless of what direction it was in. A 0.125" offset amounts to about 4 degrees of scope roll, depending on the height of the scope above the bore. An angle of 4 degrees is less than one minute of the clock, which is pretty difficult to gauge when looking at the scope from the rear of the receiver. Eyeballing it will not guarantee that you can get the alignment error that small. </p><p></p><p>For custom rifles with high quality barrels attached to receivers that have been trued, and rings that have a low lateral offset between the rail and ring axis, the use of a level on the rail followed by a level on the scope or a plumb bob down range should get the scope reticle or elevation axis within 0.125" of the bore. </p><p></p><p>What about factory rifles? I frequently see factory rifles with bent barrels or barrels installed crooked in the receiver. This is usually the case when a scope needs a lot of windage adjustment to zero the rifle at 100 yds. Many rifle manufacturers know that these issues don't cause large dispersion, so they stopped worrying about them. Most shooters don't know how much lateral offset there is in their rings, or whether the offset is left or right. These are cases where a reticle alignment tool like the RingTrue tool is going to give perfect results, whereas the use of a level on the receiver and the scope could easily lead to an aiming error in an unknown direction.</p></blockquote><p></p>
[QUOTE="bruce_ventura, post: 1045868, member: 34084"] I couldn’t agree more. Aside from wind and range estimation, there are four ballistic errors that can creep into long range shots: 1. Canting error 2. Spin drift 3. Reticle misalignment to the bore 4. Coriolis effect If we focus exclusively on the big one (canting error) and ignore the others because we assume they are small, we could get lucky and they could cancel each other out to some extent, or we could get unlucky and they could combine with each other to create a significant aiming error. [B]The goal of long range shooting is to minimize them all, so that we’re not relying on luck.[/B] Coriolis is the only error that can’t be eliminated ahead of time because we have to know where we are on the globe and what direction we’re shooting. The rest of these errors are relatively easy to eliminate, [U]so why not just zero them out and be done with it?[/U] 1. Canting error: eliminating canting error requires the use of a properly aligned anti-cant indicator. 2. Spin drift: eliminating spin drift error requires the use of a ballistic computer and knowledge of the bullet length and rifling twist rate. 3. Reticle misalignment to the bore: eliminating this error is easiest to do with a reticle alignment tool like the RingTrue tool from HighPowerOptics. Most other methods reduce but not necessarily eliminate the error. How small does this error need to be? In practice, I think the goal is to get the reticle and/or elevation turret axis aligned to the rifle bore within 0.125”, which is less than half of a ¼ MOA click at long range when the rifle is zeroed at 100 yds. That angular error would be too small to contribute to a significant aiming error regardless of what direction it was in. A 0.125” offset amounts to about 4 degrees of scope roll, depending on the height of the scope above the bore. An angle of 4 degrees is less than one minute of the clock, which is pretty difficult to gauge when looking at the scope from the rear of the receiver. Eyeballing it will not guarantee that you can get the alignment error that small. For custom rifles with high quality barrels attached to receivers that have been trued, and rings that have a low lateral offset between the rail and ring axis, the use of a level on the rail followed by a level on the scope or a plumb bob down range should get the scope reticle or elevation axis within 0.125” of the bore. What about factory rifles? I frequently see factory rifles with bent barrels or barrels installed crooked in the receiver. This is usually the case when a scope needs a lot of windage adjustment to zero the rifle at 100 yds. Many rifle manufacturers know that these issues don’t cause large dispersion, so they stopped worrying about them. Most shooters don’t know how much lateral offset there is in their rings, or whether the offset is left or right. These are cases where a reticle alignment tool like the RingTrue tool is going to give perfect results, whereas the use of a level on the receiver and the scope could easily lead to an aiming error in an unknown direction. [/QUOTE]
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