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Reticle Perpendicularity by Darrell Holland
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<blockquote data-quote="Darrell Holland" data-source="post: 229736" data-attributes="member: 7708"><p>Gentlemen,</p><p></p><p></p><p>Greetings, I'm back from our trip and thougth a few words may be interest to those interested in point of impact change when we cant the rifle at long range.</p><p></p><p>For this test I used a 6mm XC with 107 SMK bullets. The rifle was zeroed at 100 yds and 6.75 MOA was dialed into the scope to shoot at 385 yds. The rifle was canted left and right 5° 10° and 20° for this test. At 385 yds. the bullet impact was 4.7 inches left for a 5 degree cant and 9.0 inches for a 10 degree cant ( left hand cant) bullet impact was .7-.75 inches high as well. On a right hand cant the bullet was 3.6 inches right + 1.4" high, 10 degrees was 6.6 inches right and +1.1 inches high.</p><p></p><p>With a left hand cant of 20 degrees bullet impact shifted 15.5 inches left and was 2.6 inches low.</p><p></p><p>Keep in mind that we are changing the axis of the bore relative to the scope if it is not perpendicular. Anytime we cant the rifle slightly ( left or right) the bore moves up from BDC ( bottom dead center) causing a slighlty higher point of impact. When cants beyond 15-20 degrees ( this is a HUGE cant and would problably never occur in the field) the bullet impact is low.</p><p></p><p>Shooting and testing your rifle is the best way to convince yourself of this phenomenon, yes, sight height above bore does matter. Our test rifle was 1.75 inches above the bore.</p><p></p><p>Visualize two tubes mounted one above the other with a cross hair running thru the centerline of each tube. Now take the cross hair on the top tube and cant it 10 degrees left or right. Next, take the cant out ( making the reticle plumb) and watch the bore ( the bottom tube) move left or right changing the point of impact. Due to angular deflection the amount of error increases with the range.</p><p></p><p>Its imparative that the scope be mounted perpendicular to the bore for best results at long range. Anytime we cant the reticle relative to the bore we lose the ability for a true vertical path from our zero and sight alignment. With a canted reticle we induce left/right axis as well as gravity ( vertical drop ) and the rifle will need windage and elevation to shoot past our initial zero.</p><p></p><p>Hopefully this helps clear some of the muddy water concerning reticle perpendicularity.</p><p></p><p>As always be safe and shoot straight!</p><p></p><p>Respectfully,</p><p></p><p></p><p>Darrell Holland</p><p></p><p>It was interesting to note that groups canted to the right were smaller than those to the left??? The cant and bullet/barrel harmonics may be responsible for this??? Groups to the right were .830 and 1.1 inches in size. Groups to the left ran 1.3-1.5 inches... Further testing may provide answers???</p><p></p><p>It would be nice to see members duplicate the above test under near perfect conditions and let me know their results??/ I would be most interested to hear from you.</p></blockquote><p></p>
[QUOTE="Darrell Holland, post: 229736, member: 7708"] Gentlemen, Greetings, I'm back from our trip and thougth a few words may be interest to those interested in point of impact change when we cant the rifle at long range. For this test I used a 6mm XC with 107 SMK bullets. The rifle was zeroed at 100 yds and 6.75 MOA was dialed into the scope to shoot at 385 yds. The rifle was canted left and right 5° 10° and 20° for this test. At 385 yds. the bullet impact was 4.7 inches left for a 5 degree cant and 9.0 inches for a 10 degree cant ( left hand cant) bullet impact was .7-.75 inches high as well. On a right hand cant the bullet was 3.6 inches right + 1.4" high, 10 degrees was 6.6 inches right and +1.1 inches high. With a left hand cant of 20 degrees bullet impact shifted 15.5 inches left and was 2.6 inches low. Keep in mind that we are changing the axis of the bore relative to the scope if it is not perpendicular. Anytime we cant the rifle slightly ( left or right) the bore moves up from BDC ( bottom dead center) causing a slighlty higher point of impact. When cants beyond 15-20 degrees ( this is a HUGE cant and would problably never occur in the field) the bullet impact is low. Shooting and testing your rifle is the best way to convince yourself of this phenomenon, yes, sight height above bore does matter. Our test rifle was 1.75 inches above the bore. Visualize two tubes mounted one above the other with a cross hair running thru the centerline of each tube. Now take the cross hair on the top tube and cant it 10 degrees left or right. Next, take the cant out ( making the reticle plumb) and watch the bore ( the bottom tube) move left or right changing the point of impact. Due to angular deflection the amount of error increases with the range. Its imparative that the scope be mounted perpendicular to the bore for best results at long range. Anytime we cant the reticle relative to the bore we lose the ability for a true vertical path from our zero and sight alignment. With a canted reticle we induce left/right axis as well as gravity ( vertical drop ) and the rifle will need windage and elevation to shoot past our initial zero. Hopefully this helps clear some of the muddy water concerning reticle perpendicularity. As always be safe and shoot straight! Respectfully, Darrell Holland It was interesting to note that groups canted to the right were smaller than those to the left??? The cant and bullet/barrel harmonics may be responsible for this??? Groups to the right were .830 and 1.1 inches in size. Groups to the left ran 1.3-1.5 inches... Further testing may provide answers??? It would be nice to see members duplicate the above test under near perfect conditions and let me know their results??/ I would be most interested to hear from you. [/QUOTE]
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