As long as the cant angle is the same for every shot, it won't cause horizontal shot stringing.
Sine of cant angle (degrees) times bullet drop at target range equals horizontal shot displacement from zero cant.
Most precise way to see if scope elevation axis is parallel with bore to scope axis is to use a collimator in the muzzle. If it's reticle tracks parallel to the scope vertical wire, it's good.
Using a plumb line requires it be parallel to the bore-scope axis. How is that done? Twisting rifles to align scope reticles to plumb lines twists the bore - scope axis the same amount. All things have to be in line on the true vertical axis using plumb lines.
I see what you are saying. What I had figured is (and obviously suffering from probs you point out) with scope mounted and gun set up on tripod I used the plumb line to line up crosshairs, (well, the vertical one anyway) then set a small vortex level to where bubble is in the middle.
Walking in front of gun and looking from the front, I could see the magazine was ever so slightly not straight up and down. (AR type rifle) Going back behind the gun from firing position, I could see the bubble slightly off. By an ever slight pressure on the pistol grip it moves back to the middle "level" position.
This was what had me interested in this "cant" business. If I shoot from a bag, at 100 yards I seem to have a problem keeping the gun from moving slightly from side to side, with a scope. With a bipod this doesn't happen for me, much more steady. This is what led me to ask this cant question.
Bart, I will study a little on "collimator in the muzzle"......Have no idea!
Would it help to use a small level I have seen for similar purposes that attaches to the picatinney rail leveling gun, (well, the upper receiver anyway) then use plumb line to set vertical crosshairs? Then put the vortez level back on the scope tube and remove the picatinney rail level?
I assume this for cannons is not it!
https://www.seilerinst.com/division.../collimator-assembly-muzzle-reference-sensor/
Should I just buy a bore sighter, or does this present the same problem a different way?
Russellc