Bubble level and angle cosine indicator for long range shooting

To be more consistent with long range shots, deleting small errors is the key. I've participated a few sniper competitions and what I have learned from them, is that when adrenaline is pumping and there's a tight time limit for taking your shot, it's easy to cant your rifle. As a rule of thumb, 1° cant will move your point of impact 0,02 mrad for every 100 meters.
So, if cant your rifle 4°, and the target is at 800 meters, you'll miss by 4 * 0,02 mrad * 8 = 0,64 mrad. From the center of the target 0,64 mrad at 800 meter is a miss by 0,512 meters. For a human size target, what is most commonly used in competitions, thats clearly too much.
Here's a good article about cant: http://www.riflescopelevel.com/cant_errors.html

A solution for cant problems is a bubble level for your scope. Here you can see a bubble level combined with an angle cosine indicator.

 

Angle cosine indicator is used for shots that are pointed upwards or downwards. As an example, if you are shooting uphill and your slant range to target is 400 meters, it's not the range you should use for your elevation correction. As strange as it sounds, you would miss high of your target. Here's an "Improved Rifleman's Rule", which is simple and accurate enough for use in the field.

1. Measure the inclination angle of the target above or below the horizontal direction.
2. Measure the slant range distance to the target.
3. Take the Bullet Path (or come-up or come-down) from the level trajectory at a horizontal distance equal to the slant range distance, and multiply that parameter by the cosine of the inclination angle.
4. Use the result of this calculation to adjust the aim of the gun at the inclined target.

So using the ACI, you get a cosine of 0,8 for the inclination angle. For my rifle and cartridge, elevation for 400 meters is roughly 2,4 mrad. The get the hold for a shot uphill, I must hold 0,8 * 2,4 mrad = 1,92 mrad.
There are other methods too for figuring out holds for inclined fire, here's a good article about that.
http://www.exteriorballistics.com/ebexplained/article1.html

Here I'm setting up the bubble level. This is the first phase, getting it roughly level with the scope. For fine tuning, you should hang a 1 inch or thicker rope with a weight at 100 meters or so, to get it to point directly to ground. Then with good sand bag support, align your scopes vertical line to the rope. When the vertical line follows the rope nicely, check your bubble level. If it isn't level, loosen the screws and adjust.