Calculating windage error due to rifle cant

[Not_A_Llama]

Was down at the PD match on Saturday and got to thinking about this, so I did the math.

BD = Bullet Drop
A = Angle rifle is canted from vertical
E = error, left or right

E = BD tan A

Example:
At 425 yards, my .308 drops 31" with a 100 yard zero. If my rifle is canted 5° from vertical, the windage error is:

E = 31" X tan 5°

E = 31" X 0.0875

E = 2.71"

Here's another example because 5° is a lot and 425 yards isn't.

Same rifle at 1,000 yards, bullet drop is 321". Assume cant of only 2°, which seems more realistic.

E = 321 tan 2°

E = 11.2"

With your way of viewing things, I think you're actually looking for the Sine, not the tangent - consider the behavior of tangent at 45 degrees.

There's actually more calculations that go in, and there's a reference point problem - it's a cool problem, I'm writing more up. My basic contention of there being a problem is that the way you're modeling it basically looks like 1) sight in 2) fire 3) rotate 4) fire 5) measure difference, while the more "accurate" behavior looks like 1) sight in 2) fire 3) rotate 4) sight in whilst canted 5) fire 6) measure.