Bubble levels - carpenters have been using bubble type levels for 100's of years - simple devices.
I paid about $10 for this one at a local pawn shop - works great, most likely die cast aluminum, I lapped the inside to improve contact with scope tube. I mounted it and use it just like as described by ShootnMathews ("I can look through the scope with my right eye and see the bubble with my left eye without moving my head").
Mounting the bubble level on the scope correctly is essential and the level needs to coincide (correspond in position; meet or intersect) with the reticle, like don't depend on using the top of the elevation dial as a reference.
Knowing the consequences of scope cant and how to calculate cant error is helpful. A 1-degree cant will cause a lateral error of almost 2 inches with an 87 VMax at 3450 fps at 650 yards.
Understanding rifle cant includes understanding physics, but the most errors come from not holding the rifle bore axis and scope axis on a vertical plane
www.ssusa.org
Knowing how to precisely mount the bubble level to coincide with the reticle is essential:
ACCURATE SCOPE MOUNTING MADE EASY A level reticle is critical to long range Accuracy. For longer shots a canted (or unlevel) reticle will pull or push the bullets trajectory off of vertical center. It gets worse the further the distance increases. A level scope means that the reticle...
www.realavid.com
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