Originally Posted by X-man
Are you sure you need one?
This might rub some folks the wrong way, but try canting your rifle slightly and tell me what happens?
Bottom line is extremes will move the POA, but minor errors do not.
I partly agree with what you're saying but it boils down to two things.
For small angles the effect of cant in inches is sine( cant angle) * (192)* T^2 where T is the time of flight of the bullet.. (192 is 1/2 the acceleration of gravity in inches per second squared) This is a horizontal error always perpendicular to the gravity vector. . There is a vertical error component but its still negligible even at 10 degrees of cant By using time of flight the bullet velocity and distance to the target don't matter. The numbers are cant angle versus inches of horizontal error at the target.
T of flight 1 sec 2 sec
1 degree 3.26" 13.0"
10 degree 33.3" 133.3"
Few people hunt game where the range and rifle give 2 second time of flight. Arguably 1 second time of flight is "long range" for most hunters.
But how well can you estimate how canted your rifle is? While humans have the vestibular system of the inner ear which is essentially a set of acceleration sensors that only gives a precision of about 10 degrees for the static angle of the gravity vector. It's only one of several sensors the body uses to sense vertical. The sensors aren't directly perceived by one's consciousness. You only get a feeling of "upright" or "tilting" or "tilted" and that comes from a mix of the available sensors.. When a person is sitting or prone there are better signs of vertical from the forces on whatever parts of their body are supported if a person has no visual visual. With a little practice a person standing may be able estimate vertical without fisual clues to one degree from the the pressure on their feet if there are no other forces than gravity on the body, but light wind pressing on the body can add considerable error.
Visual clues are by far the strongest in determine a person's sense of vertical. Being able to see a horizon, standing trees, human made structures, and bodies of water are very strong clues which override other senses even if they're wrong. In mountainous terrain visual clues are often nonexistent or and sometimes present but wrong.
You may not need a level for the distances or the terrain where you shoot. On the other hand it's an error source which can be completely eliminated by using a simple bubble level. It's cheap and easy to to use compared to other sensors like rangefinders and air density meters. If someone would market an instrument which could measure >downrange< crosswinds and calculate the resulting bullet deflection the only error sources left which cannot be easily controlled would be velocity variation of the ammo and the stability of the shooter to hold the rifle steady while pulling the trigger.