Ok , so when using a plumb line
How can we makesure the rifle is level to begin with?
Kyle
Let me ask you this, What does it matter whether the
rifle is level or not?
As long as the scope is level, elevation adjustments will only affect elevation, regardless of how canted the rifle is.
If the scope is canted, the crosshairs will move down and to one side just by cranking only the elevation knob.
If the rifle is canted and the scope is level, you will have a very very small horizontal shift, but it will be a fixed ratio. For example, the M1D Garand. The scope is mounted roughly 2" to the left of the bore center axis (as well as a fair bit above it, but since our scope is level, the elevation turret fixes that and we need not worry about it). I don't know exactly how far left of center, so I will just use 2" for this example. At point blank, POI is 2" right of POA. At your 100yd, POI is 1" right of POA, at your 200yd zero, POI is POA, at 300yd, POI is 1" LEFT of POA, at 400yd, POI is 2" left of POA. etc. etc. At 800yd, POI is 5" left of POA.
Let's look at the other side of things. Assume your scope is 3" directly over your bore, zero offset, however your optic is canted 5 degrees. I'll use generic ballistic data, .308 175smk @ 2550fps 29.92 in/Hg, 59 degrees.
At 800yd, I'll save you the trig, but at 5 degrees offset, your bullet will hit just a couple inches low, and 19-20" to the left/right of where you were aiming.
So yeah, ideally your scope would be level, and directly over your rifle's bore. That doesn't necessarily mean your rifle is level, and the way you hold your rifle, it may not be comfortable to hold it directly level. The point is that it's really not that big of a deal (extreme example of 5/8MOA shift at 800yd). What's important is that the scope is level.
edit: I'll be even more specific. The elevation turret is what you want leveled to gravity, because it directly adjusts for gravity's pull on the bullet. Everything else is an after-thought. That is, unless you use the reticle for hold-overs, or a BDC type reticle and don't touch the turrets, then it's important to level the reticle. It's very possible that the reticle and turrets are square to eachother, but I don't make that assumption until I've proven whatever scope I'm using.