All Springer/NP/PCP Air Gun Discussion General > Optics, Range estimation & related subjects
Cant errors
rsterne:
If the lower extension of the vertical reticle is through the center of the barrel, and that line is also vertical when you shoot, then nothing else really matters.... In fact, the action (and stock) could be laying on it's side, with the scope mounted on the side of the rifle, but vertically above the boreline and the gun would still shoot perfectly.... It might look weird, but no cant error would occur.... I think I'm going to make a device to insure all future scope installations will have the vertical scope reticle passing though the center of the boreline.... As you say, that's the starting point....
Something like this.... http://www.projectsavage.com/?q=content/exd-engineering-vertical-reticle-instrument
Bob
Scotchmo:
Bob,
It sound like you now have a good handle on the concepts.
There are tools that are made for this purpose. I just use a mirror set at 5 to 10 yards down range to do the same. Either will get you very close. I can correct for most of the scope cant. But unless the reticle is perfectly centered on the objective bell, there may still be a slight cant. The ultimate verification is while shooting various distances at the range.
Mirror method:
I stand a mirror down range so that I can look through the scope and see the scope objective and the end of the bore in the mirror. I then rotate the scope in the mounts until the crosshair in the scope intersects the center of the bell housing and the bore at the same time. That gets me in the ball park. On low, one piece mounts, the bottom of the turret housing can sometimes interfere with the mount before the scope can be rotated enough. On at least one of my rifles, I just live with the minor cant error that remains. The better way might be to machine some additional clearance into the mount.
When I say "gun" cant, I'm referring to that vector between the center of the scope and the center of the bore. The stock, the grip, the scope mounts - none of that matters in the final determination of cant. The stock and scope mounts can be twisted and warped any which way, and as you said, even sideways.
I use the mirror to correct the "scope cant" and then lock down the scope mounts, then I can use a plumb line (or carpenters level or edge of a building) to adjust the bubble level and tie it to the reticle. Then, when I am in the field, I use the gun mounted bubble level to eliminate "gun cant".
"Scope cant" happens from an incorrectly mounted scope. "Gun cant" happens when I don't hold the "gun" perpendicular to the earth.
Scotchmo:
The following is a procedure for eliminating both types of cant errors.
Adjust out any scope cant (important for any shot requiring holdover, close or far):
1) Sight in at the trajectory apex (usually 23-32 yards, 25 yards is good)
2) At 10 yards, shoot at a target that has a vertical line on it. Use a plumb line or construction level to insure it's vertical.
3) Rotate the scope in the mounts until all shots hit exactly on the line at 10 yards.
4) recheck the 25 yard zero and repeat until zeroed at 25 yards and on the line at 10 yards.
After that, your scope should be locked down and securely tightened in the rings. If you plan on using a gun mounted bubble level, you still need to adjust that.
Eliminate gun cant (especially important for far shots):
1) line the reticle up with the vertical line on the 10 yard target (or use a plumb line at any distance).
2) Rotate or adjust the gun or scope mounted bubble level until the bubble indicates level while the reticle is still on the plumb line. Lock the bubble level in that position.
3) Make sure the bubble indicates level whenever you shoot.
There is not much point in eliminating gun cant until you are confident that the errors are not related to scope cant. So, adjust out the scope cant first. Scope cant is eliminated by adjusting the scope in the rings. And then gun cant is eliminated by holding the gun and scope level.
Powder burner:
Good stuff. Thanks for the info, Scott.
calixt0:
--- Quote from: Scotchmo on August 04, 2014, 01:37:09 PM ---The following is a procedure for eliminating both types of cant errors.
Adjust out any scope cant (important for any shot requiring holdover, close or far):
1) Sight in at the trajectory apex (usually 23-32 yards, 25 yards is good)
2) At 10 yards, shoot at a target that has a vertical line on it. Use a plumb line or construction level to insure it's vertical.
3) Rotate the scope in the mounts until all shots hit exactly on the line at 10 yards.
4) recheck the 25 yard zero and repeat until zeroed at 25 yards and on the line at 10 yards.
After that, your scope should be locked down and securely tightened in the rings. If you plan on using a gun mounted bubble level, you still need to adjust that.
Eliminate gun cant (especially important for far shots):
1) line the reticle up with the vertical line on the 10 yard target (or use a plumb line at any distance).
2) Rotate or adjust the gun or scope mounted bubble level until the bubble indicates level while the reticle is still on the plumb line. Lock the bubble level in that position.
3) Make sure the bubble indicates level whenever you shoot.
There is not much point in eliminating gun cant until you are confident that the errors are not related to scope cant. So, adjust out the scope cant first. Scope cant is eliminated by adjusting the scope in the rings. And then gun cant is eliminated by holding the gun and scope level.
--- End quote ---
never thought of doing it this way.. genious
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