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Canted scope, canted rifle...

 
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Old 12-30-2005, 12:56 AM
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Canted scope, canted rifle...

I'm going to pull some stuff over here from another forum I frequent. I hope these issues are clear enough, but if not, feel free to ask for clarification (or take issue, if you feel so inclined). [img]/ubbthreads/images/graemlins/smile.gif[/img]



This picture represents a canted rifle underneath a level scope. You're looking at the buttstock of the rifle, and on through the scope. The orange circle is the approximate location of the bore of the rifle. The blue dots represent the fall of the shots as they go downrange; they fall with gravity along a line which is represented by the vertical crosshair in the scope.

What we must concern ourselves with to understand this issue is the scope and the bullet path. When you dial the scope's erector to zero for windage, you are essentially aligning it with the bullet path--not the barrel!

So if a scope is mounted slightly canted, but held level, the bore of the canted rifle would only be off to the side a fraction of an inch (perhaps 1/16 to 1/8 inch) underneath. It would look something like this:



Is it important that the bore isn't 100 percent underneath the scope's vertical crosshair? Actually, no.

Think about it this way:

If you have the scope dialed to a perfect 100 yard zero with one particular load, and then you switch to another load, you'll likely note that your windage zero will change. Has the scope moved? No. Has the barrel changed? No. Only the direction that the barrel is throwing the shots has changed. Barrels, by their very nature, throw shots here, there, and yonder. [img]/ubbthreads/images/graemlins/laugh.gif[/img] So you must dial the scope's erector to follow the general path of the new load to get your zero. This may take the scope's centerline well away from the boreline--but that's not what's important. Bullet path and boreline are two different things.

You see, the scope's erector is never actually aligned with the bore of the rifle to begin with--it is aligned with the path of whatever bullets you are setting the zero for.

What I'm saying is that you can have a slightly canted scope, with the barrel underneath at, say, 5:30 and so long as the scope is held level, the shots will still fall parallel with the vertical crosshair.



In the top image, there is the ideal situation where the scope's vertical crosshair perfectly disects the fall of the shots.

In the second image, the scope is mounted with a slight cant, but since the scope is being held level, this means that the rifle bore is off by a bit underneath it. It's at 5:30 rather than 6 o'clock. Note how the shots fall just slightly to the right of the vertical crosshair. Groups forming downrange would probably never indicate that the 5:30 rifle cant even existed, as these shots would only be off to the right of the vertical crosshair a tiny fraction of an inch.

What if you layed the rifle on its side?

If you layed the rifle on its side (and mounted the scope upright, with the elevation turret up top),



...this would of course put the scope about 1.5 inches to the left of the bore. If you zeroed the scope for the shots to fall dead on at 100 yards then yes, you would have an angular relationship with bullet path and line of sight. You'd only be zeroed for 100 yards. From the rifle to the target, you'd begin with almost 1.5 inches of error, slowly correcting until you got to 100 yards, then beyond 100 yards your shots would deviate farther and farther from the windage zero you had at 100 yards.

However, if, as is shown in the drawing immediately above, you were to take into account that 1.5 inch difference with the "sideways rifle," and you dialed the scope so that the shots fell 1.5 inches to the right of the crosshair intersection at 100 yards, these shots would stay only 1.5 inches right of the line of sight all the way downrange (wind factors and such notwithstanding).

In another possible scenario, you could simply take this sideways rifle and dial the windage zero to be correct at 1000 yards. Then, you'd be off a little less than 1.5 inches at 100 yards, and the bullet would begin "closing in" on the windage--and the closer the bullet got to 1000 yards, the more it would close the 1.5 inch gap. At 1000 yards, the bullet would cross the line of sight, then begin deviating in the opposite direction, and by 2000 yards, it would be about 1.5 inches to the other side. Obviously, this 1.5 inches would not be much of a factor at the longer ranges. [img]/ubbthreads/images/graemlins/smile.gif[/img]

So, with the slightly cant-mounted (but held level) scope, you're not going to be off anywhere near 1.5 inches all the way downrange. It'll be more like 1/8" or so--and wouldn't even be noticed in a 1 MOA group size.

So remember: It's bullet path, bullet path, bullet path--not the barrel that you're aligning the scope with.



The above is a top view, looking down at the rifle and scope, and bullet path(s). The blue area would represent the scope's range of WINDAGE alignment; in other words it can be dialed to windage zero anywhere in that blue zone. The orange lines represent the various paths different load recipes might cause the bullets to fly. You can dial the scope to align with any of these paths.

All that really matters is that the bullets be released relatively close to the vertical crosshair and all will be well.

When the barrel releases the bullet, the bullet goes up, then it comes down. It does this regardless of where the barrel is in relation to the scope. All you are doing with the scope windage adjustments is aligning the erector to be closely parallel with that bullet's path. The key words here are CLOSELY PARALLEL, and once the scope's windage is set to be parallel with the bullet's path, you will not have to change windage for various ranges--it'll stay the same--even if the barrel is at 5:30 or 6:30, or even 5 o'clock or 7 o'clock underneath the scope. You might get lucky and have your bullet fall perfectly along the vertical crosshair of your scope, but that'll be the exception; not the rule. Chances are, if you could somehow determine the "perfect bullet" path, it would be 1/8" or even more, to one side or the other of the vertical crosshair. Big deal, though--such a small amount of error won't be noticed on the targets.

The problem that some of us may have in understanding this is we are assuming that bullets eminate in a straight line from the bore of the barrel--which they do not. Some bores are not even drilled straight through the barrel, so you could have a scope centered "perfectly" over the barrel and still have a "canted bore." But relax. Hold your scope level for each shot and you'll still shoot just fine.

I have a ScopLevel on a Bushnell Elite scope. The level is mounted to the tube of the scope, and I move that scope from rifle to rifle as I test different guns and different loads. It's currently on my recently rebarreled Remington 788, and it is not perfectly straight with the receiver. I noticed this when I put the rifle on a bench at our rifle range. However, since I began testing that rifle, I have hit varmints at 200 yards, 540 yards, and 755 yards by making elevation changes only. I did not make windage changes for these shots because the wind wasn't at issue. A couple weeks ago, again, without touching the windage turret, I dialed in my elevation for 1050 yards and shot at this target:



While the group has moved a tiny bit to the right, that is to be expected at such long ranges as the bullet's spin causes it to move slightly aside.

Paralleling the scope's line of sight with the bullet's path is really no big deal.

Keep the scope level, and don't worry so much about whether it's perfectly straight up and down over the bore. It's nice, yes, to have a straight scope. But if your scope is slightly canted in the mount, just level the crosshairs as you normally would, and you'll have no trouble at any range.

One last illustration, which I hope will bring it all together... [img]/ubbthreads/images/graemlins/smile.gif[/img]




Dan Newberry
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  #2  
Old 12-30-2005, 03:58 AM
ds ds is offline
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Join Date: Oct 2004
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Posts: 235
Re: Canted scope, canted rifle...

Green 788,

Thanks for posting that. One of the best things I have read in a long time.

As it happens I am mounting a scope and it helped me to understand a few things.

David.
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Old 12-30-2005, 07:00 AM
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Join Date: Nov 2004
Posts: 27
Re: Canted scope, canted rifle...

Green 788,
I really appreciate the education. It got the old brain cells activated this morning.There were several things that I didn't think of. Thanks again. [img]/ubbthreads/images/graemlins/smile.gif[/img]
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