How do you align it to the barrel?
The requirements to eliminate cant error are:
1. Align the scope's reticle with the rifle's barrel.
2. Hold the reticle level when shooting.
Item 1 deals with scope cant...i.e. mounting the scope incorrectly.
Item 2 deals with gun cant...i.e. holding the gun incorrectly.
Here's the easiest way to eliminate cant errors. The great part is you don't need any special tools. All you need is a mirror. Set the mirror at a distance of, say, 5 yards and set your AO to 10 yards. Look through the scope at your reflection in the mirror. Twist the scope in the mounts until the vertical bar of the reticle simultaneously bisects both the muzzle and objective bell. Then lock it down. At this point you have eliminated scope cant (item 1). If it's hard to see your muzzle, add a little dot of White Out or take a dot from a hole punch and tape it to the muzzle with clear tape.
Now to eliminate rifle cant, install and use a level. View a known good plumb line (e.g. hanging string) through the scope and orient the rifle so it precisely aligns to it. Now affix your level so it shows level. From now on when you're shooting, hold the rifle so it shows level and you can be sure you've eliminated both potential sources of cant error.
By the way, don't buy a level that attaches to the scope rail. There is no guarantee it will show level. Instead, get one that attaches to the scope tube so you can rotate it and lock it down where it shows the proper level.
