Leveling the reticle to the gun is not necessary UNLESS you have elevation built into a scope base. I.E - a 20MOA pic rail. If you have a 20MOA rail like most long range guys are using then it is ABOSOLUTELY necessary to make sure the reticle is not only plumb to the scope base/turret and the Earth by using a plumb bob, but also to the gun or whatever device that has elevation built into it.
Failure to do so will turn elevation into windage and windage into elevation when dialing and will only get worse with distance.
In theory, yes. In practice, no. Scope bases, rings, all the little pieces inside a scope that no one knows exactly what they are and do except optical engineers all matter more.....and....more than that.....the barrel....wait for it.....it's not....perfectly straight. Wait, what????!!!!! The barrel is bent way more than the miniscule amount a tiny bit of cant is going to cause with your 20 or 30 MOA scope base. Make the erector travel plumb and level and the scope will work. If you want, you can shoot the gun sideways. If you mount the scope with the vertical turret pointing the same way as the ejection port, put a bubble on the tube, and make the bubble centered when the erector travel for up/down is perfectly plumb the error is very easy to quantify. The horizontal offset (which would be height over bore in a normal configuration) is easy to account for. If your height over bore is 2" and you zero the gun to shoot 2" to the right of point of aim, it will hit 2" right from zero to infinity barring the other effects of windage. You can then use your turrets to account for drop as normal and they will function exactly the same as if the scope was mounted with the reticle perfectly plumb above the bore.
You could also zero windage to be on at an arbitrary range. Say you zero windage at 1000 yards. Your zero will be off 2" at the muzzle, get closer to zero until 1000 yds and then start diverging until it is 2" off in the other direction at 2000 yds. This is with the gun canted 90 degrees. The offset can be calculated and accounted for with a ballistic calculator in order to actuallyshoot the gun on its side. There are other real life things that can effect it like the barrel being bent, muzzle sag, barrel whip, and other things that we ALL compensate for by twisting turrets.
In addition, if you take this extreme example and assume it's all unaccounted for, unknown error, with a 100 yard zero and a 2" unaccounted for cant, the error at 1000 yards is 18" or around 3mph wind.
No one is unknowingly canting their scope 2" to the side of the bore. As I stated in my first post. The theoritcal is very real. In practical application you will not shoot the difference.
Another strong argument for my method is this. Say you naturally cant the gun 2 degrees and your height over bore is 2". You set the scope up based on level gun/level scope. You dial on 10 mils to shoot 1000 yds. You level the scope and while you aim and squeeze you float back to that natural 2 degree cant. You're now aiming off over 12.5" horizontally.
My method. Worst case. You zero at 100 yards. Your horizontal offset from a 2 degree cant is 0.069799" or just over 1/16". You naturally hold the reticle level. The offset at 1000 yards with 10 mils of drop (because we zero at 100 yards and it diverges from there) is 0.628191" or just over 5/8". Even if I screw up and cant the gun 2 more degrees my miss is 13 1/8" and your miss is 12 17/32".
There is no reason to buy a bunch of tooling and do a bunch of time consuming scope leveling to guns. Use feeler gauges or playing cards if you want it reasonably close to satisfy your ocd. Then put a level on the scope tube and make the reticle and erector travel plumb to gravity and center the bubble.
I am sorry for being flippant, but people want to make the easy stuff hard and the hard stuff easy when it comes to long range shooting. I wish I could get back the hours I spent agonizing over stuff that doesn't matter. This is one of those things that is counterintuitive in the real world.
