Re: 90 degree left or right cant
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Graham</div><div class="ubbcode-body">I wouldn't suggest setting up the scope for it, because that will screw up your zero.
Assume that we’re talking about a 100 yard shot:
The elevation zero is comprised of two parts: First is compensation for the bullet drop from the muzzle to 100 yards. With a .308 that drop is about 2.7 inches, or 2.6 MOA or .75 Mils. Second is the mechanical offset of the line of sight over the line of the bore. Assume my sight height of 1.75 inches, which at 100 yards is about 1.7 MOA or .5 Mils.
Add those two up, and you get about 4.3 MOA or about 1.25 Mils.
If you rotate your rifle to the left, which is bolt-up for a right-handed rifle, your windage is now your elevation (but it’s at zero). And your elevation zero is now your windage zero, but it’s about 1.25 Mils to the right of the line of bore.
So, if you fire a shot at point of aim, with no adjustments on the scope, the point of impact will be about 1.25 Mils left of the point of aim because that is how far the scope is off from the line of the bore. And the shot will be about .75 Mils low because there is no longer a sight height to compensate for since - with the rifle sideways - the scope is at the same level as the bore.
Therefore, for this kind of shot, you hold to the right by about 1.2 Mils and high by about .75 Mils, you should be pretty close to the target.
Of course, if you are making a bolt-up shot at a distance of greater than 100 yards it gets more complicated. In that case you might want to establish a parallel bore zero. Then you would have to hold to the right by the amount of the sight-height, and hold high by whatever the actual bullet drop is at that distance.
[Thanks to Lindy for explaining all this to me a long time ago, before I really understood it.
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Wow, that really got me thinking about this. It makes sense but I have a few comments / questions?
1) If you were only shooting at 100 yards and wanted the left/right point of impact (POI) to match the point of aim (POA), then I would think you only need to lower the elevation by the gravity component (2.6 MOA or .75 mils per the example). This would leave the mechanical offset of 1.75" to match the POI to the POA (at least horizontally).
2) To get a parallel scope line of sight to the bore, in the right hand bolt up configuration, wouldn't you need to remove both elevation components discussed above? So lower the elevation knob (before you roll the rifle) by 4.3 MOA or 1.25 mils per your example? I would think so...
3) With the parallel bore case, and the right hand bolt up configuration, the POI would be 1.75" RIGHT of the POA, since the scope is 1.75" left of the bore. So you would have to hold 1.75" LEFT, not right. Note, that this 1.75" is an actual distance unit, not angular unit like IPHY, MOA, etc.
So if I've thought about this correctly, here are the steps I'd take to get a parallel bore, and get the proper holds.
(for the right hand bolt up configuration)
1) Dial elevation DOWN for the scope height and the gravity drop for your zero. 4.3 MOA or 1.25 mils per your example. This gets the parallel bore to scope.
2) Dial wind RIGHT for the gravity drop for your zero. 2.6 MOA or .75 mils per the example. This gets your new elevation set for your zero distance.
3) Roll rifle left, bolt up for right handed bolts.
4) Hold 1.75" (or whatever your scope offset is) LEFT via the reticle.
5) Hold UP for shots past your zero based on your standard dope , or dial via the original wind knob (dial RIGHT).
I guess you'd have to remember where your 'zero' is on your wind knob (it probably won't be on the printed 0), if you are going to dial for elevation. Remember how many rotations you've made, etc...
Seems like a lot of work. How often are these shots mandated at a competition? I've never heard of one.
