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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Help Needed with LB3.0 and .338 Bergers
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<blockquote data-quote="Pdvdh" data-source="post: 398000" data-attributes="member: 4191"><p>I obtained the effect of substantial L to R drift with one 7mm Rem Mag rifle because the scope wasn't mounted parallel to the bore in the left/right plane. This kind of thing doesn't play a significant role until the target or game are engaged at the farther ranges we find challenging. I didn't know the scope's alignment was off enough to create the excessive L to R drift until shooting at 1000 yds with it and my 300 Win Mag at the same time one afternoon. Both rifles sighted about 3" left @ 300 yds to allow for Coriolis and spin drift @ 1000 yds. The 7mm Rem Mag drifted quite a ways rightward compared to the 300 Win Mag. Too much to be explained by combined Coriolis and spin drift. I wrote this experience up in a Thread on this Forum back in 2008. This was the beginning of the research effort for me. The quest to get my arms around these mysterious causes of drift.</p><p></p><p><a href="http://www.longrangehunting.com/forums/f17/left-300-yds-right-1000-yds-wth-35255/" target="_blank">http://www.longrangehunting.com/forums/f17/left-300-yds-right-1000-yds-wth-35255/</a></p><p></p><p>To cut to the chase; I finally removed the scope, ran a long straight edge along each side of the scope rail and observed the gap between the straight edge (a 4' level works as a straight edge) and the muzzle. Along one side of the scope rail, the straight edge was over the middle of the muzzle. Along the other side of the scope base rail, there was ~ 5/8" gap between the side of the barrel and the straight edge. I loosened the two-piece scope base, re adjusted those mounts to get the straight edge running parallel to the bore, remounted the scope, headed back out to the 1000 yd target - Problem Solved. Now the L-R drift was about the same as with my 300 Win Mag. Then when I ran the dope through LB, the L-R drift I was observing was precisely accounted for with the combination of spin drift and Coriolis drift.</p><p></p><p>Now I always use one-piece scope bases, I always use the straight edge to confirm my scope base is running parallel to the bore, I always level the bore and scope's objective plumb with gravity using a tool/device obtained from Brownells, I always mount an anti-cant level to the scope rail or scope tube, after using the Brownells tool I always hang a plumb bob down the driveway and then rotate the scope tube until the vertical cross hairs are overlapping (in vertical alignment with) the plumb bob string, and I always then note the position of the level bubble (reading) on my anti-cant device. If it's mounted on the scope tube I rotate the device to level the bubble when the vertical crosshair overlaps the plumb bob string. If my anti-cant device is mounted on the scope rail I will mark new bubble level lines to calibrate the bubble to the real world level conditions I just established when the scope's vertical crosshair overlaps the plumb-bob string (if the unit's bubble calibration lines are incorrect). In other words - everything is plumb to the bore in the vertical direction, aligned with the world and aligned parallel with the force of gravity. Since adopting these standardized scope mounting practices, I have no surprises with either a lack of spin drift or coriolis, or an excess of spin drift or coriolis. </p><p></p><p>I accept that coriolis is always in play. If Isaac Newton and Albert Einstein proved it, I accept it. Believing or not believing makes no matter to Coriolis drift. Should the world stop spinning, then I'll forego the dope for Coriolis at long range. It's a proven fact that it is affecting POIs at long range unless you live, shoot, and hunt at the Earth's equator. </p><p></p><p>I also accept that spin drift is in play with every rifle I've zeroed at 300 yds and fired in windless conditions at 1000 yds. It's admittedly less predictable, more variable, and consequently more difficult to model than Coriolis. But modeled dope for spin drift is almost certainly going to result in a closer POI than no dope at all.</p><p></p><p>Factors I've experienced and/or am aware of that could offset Coriolis and spin drift (L to R drift) without shooter awareness are 1) canting of the rifle to counteract these drifts, 2) mounting the scope non-parallel to the bore in the left to right plane which will serve to either counteract or exacerbate apparent drift, and 3) crosswinds which can add to or subtract from these two sources of drift. If others know of additional masking factors, please sound in.</p><p></p><p>I've heard the opinion more than once that wind doping so grossly controls L to R dope that Coriolis and spin drift get lost as background noise compared to wind drift. But then at least one member with that opinion admitted he lived in tornado alley. The strong prevailing winds where he shot didn't make it worth the bother to dope for Coriolis or spin drift. Again, with respect to other's findings and preferences, I say live and let live. </p><p></p><p>The sport is intended to be enjoyable. Don't ruin it by making it overly complicated if you dislike complication. Or if you enjoy the additional complication in doping for mysterious drifts, than add LB to your arsenal and factor in Coriolis and spin drift. If you don't see them on target it's not because they aren't in-play. They're occurring surely as the sun rises in the East and sets in the West.</p></blockquote><p></p>
[QUOTE="Pdvdh, post: 398000, member: 4191"] I obtained the effect of substantial L to R drift with one 7mm Rem Mag rifle because the scope wasn't mounted parallel to the bore in the left/right plane. This kind of thing doesn't play a significant role until the target or game are engaged at the farther ranges we find challenging. I didn't know the scope's alignment was off enough to create the excessive L to R drift until shooting at 1000 yds with it and my 300 Win Mag at the same time one afternoon. Both rifles sighted about 3" left @ 300 yds to allow for Coriolis and spin drift @ 1000 yds. The 7mm Rem Mag drifted quite a ways rightward compared to the 300 Win Mag. Too much to be explained by combined Coriolis and spin drift. I wrote this experience up in a Thread on this Forum back in 2008. This was the beginning of the research effort for me. The quest to get my arms around these mysterious causes of drift. [url]http://www.longrangehunting.com/forums/f17/left-300-yds-right-1000-yds-wth-35255/[/url] To cut to the chase; I finally removed the scope, ran a long straight edge along each side of the scope rail and observed the gap between the straight edge (a 4' level works as a straight edge) and the muzzle. Along one side of the scope rail, the straight edge was over the middle of the muzzle. Along the other side of the scope base rail, there was ~ 5/8" gap between the side of the barrel and the straight edge. I loosened the two-piece scope base, re adjusted those mounts to get the straight edge running parallel to the bore, remounted the scope, headed back out to the 1000 yd target - Problem Solved. Now the L-R drift was about the same as with my 300 Win Mag. Then when I ran the dope through LB, the L-R drift I was observing was precisely accounted for with the combination of spin drift and Coriolis drift. Now I always use one-piece scope bases, I always use the straight edge to confirm my scope base is running parallel to the bore, I always level the bore and scope's objective plumb with gravity using a tool/device obtained from Brownells, I always mount an anti-cant level to the scope rail or scope tube, after using the Brownells tool I always hang a plumb bob down the driveway and then rotate the scope tube until the vertical cross hairs are overlapping (in vertical alignment with) the plumb bob string, and I always then note the position of the level bubble (reading) on my anti-cant device. If it's mounted on the scope tube I rotate the device to level the bubble when the vertical crosshair overlaps the plumb bob string. If my anti-cant device is mounted on the scope rail I will mark new bubble level lines to calibrate the bubble to the real world level conditions I just established when the scope's vertical crosshair overlaps the plumb-bob string (if the unit's bubble calibration lines are incorrect). In other words - everything is plumb to the bore in the vertical direction, aligned with the world and aligned parallel with the force of gravity. Since adopting these standardized scope mounting practices, I have no surprises with either a lack of spin drift or coriolis, or an excess of spin drift or coriolis. I accept that coriolis is always in play. If Isaac Newton and Albert Einstein proved it, I accept it. Believing or not believing makes no matter to Coriolis drift. Should the world stop spinning, then I'll forego the dope for Coriolis at long range. It's a proven fact that it is affecting POIs at long range unless you live, shoot, and hunt at the Earth's equator. I also accept that spin drift is in play with every rifle I've zeroed at 300 yds and fired in windless conditions at 1000 yds. It's admittedly less predictable, more variable, and consequently more difficult to model than Coriolis. But modeled dope for spin drift is almost certainly going to result in a closer POI than no dope at all. Factors I've experienced and/or am aware of that could offset Coriolis and spin drift (L to R drift) without shooter awareness are 1) canting of the rifle to counteract these drifts, 2) mounting the scope non-parallel to the bore in the left to right plane which will serve to either counteract or exacerbate apparent drift, and 3) crosswinds which can add to or subtract from these two sources of drift. If others know of additional masking factors, please sound in. I've heard the opinion more than once that wind doping so grossly controls L to R dope that Coriolis and spin drift get lost as background noise compared to wind drift. But then at least one member with that opinion admitted he lived in tornado alley. The strong prevailing winds where he shot didn't make it worth the bother to dope for Coriolis or spin drift. Again, with respect to other's findings and preferences, I say live and let live. The sport is intended to be enjoyable. Don't ruin it by making it overly complicated if you dislike complication. Or if you enjoy the additional complication in doping for mysterious drifts, than add LB to your arsenal and factor in Coriolis and spin drift. If you don't see them on target it's not because they aren't in-play. They're occurring surely as the sun rises in the East and sets in the West. [/QUOTE]
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