You missed all the posts that mentioned a scope level to reduce errors associated with cant.
I haven't missed anything. That device that you just posted a picture of is completely useless if the steps before it aren't level. There are many steps needed before you glance at your tacticool scope level, if you're doing it for anything more than to impress chicks.
Step zero: this concept is almost completely irrelevant under about 600yds.
Step one: You have to get the action level to the horizontal (good f-Ing luck - even the level-level-level system assumes that the rail is level to the action. It may or may not be.)
Step two: you have to get the rail level to the previously levelled action. That pic rail is about 3/4" across the top. That 3/4" surface area is a pretty narrow balancing area for your harborfreight/princess auto level to sit on.
Step three: you have to get your rings/mount level to the previously levelled rail and action. Let's assume that millett/ATRS/weaver/vortex/NF/alpha/SPUHR are on their game, at all times, which we know they aren't. None of us are.
Step four: you have to get your reticle TRULY level/perpendicular to the previously levelled rings/mount, which was previously levelled to the rail, which was previously levelled to the action. This is where my Frankensteinlevel device comes in....
Step five: you have to now test your scope for tracking, at a long range (ie. 600-1000yds) to see if your optic is as true as you hope it is by dialling up 10-20-30 MOA and tracking your hits one a 10' tall piece of paper.
Step six: you have to do your job, as a shooter, better than you ever thought possible to even be worrying about this minutia. This mental masturbation applies to few, and most days i'm not one of them.
GGG