Advanced Marksmanship Where on the rifle do you measure scope height?

[NAProf]

Hi Guys,
I am new here. I am an air rifle shooter who is looking at precision shooting at long ranges for an air rifle (150-200m). Scope height is a very important input with the very low BC's for airgun pellets (and even slugs). I have was taught that I should measure scope height at the scope objective lens. But my rifle has a 30MOA scope rail. If I measure scope height at the scope objective I am not accounting for the barrel up angle the rail introduces. Shouldn't I really be measuring the distance between the line of sight of the scope and the centerline of the barrel at the muzzle? After the projective leaves the muzzle gravity starts to play a role and the propulsive force goes to zero. If that is the case then shouldn't I also include any angle of the erector tube. I have notice that small changes in scope height input can affect my dope for the longer ranges.
I figured this would be the place to ask because you guys have way more experience in this stuff than I do. I am just an overly scientific retired engineer.
Cheers,
Greg

 

[mojavejim]

center of bore to center of scope at the turrets is the norm.

 

[NateVA]

I just pull the bolt back in line with the ocular. Then measure center to center.

 

[lash]

center of bore to center of scope at the turrets is the norm.

I’ve used this method with good success, even with standard velocity .22.

 

[NAProf]

Thank you, gentlemen, for taking the time to respond to my question. The takeaway for me is that because of the high velocities of your projectiles and the high BC's. there is very little projectile drop at the ranges you zero for. From Sterlok Pro with a Hornady Amax 308 168gr bullet you need to correct for a 2.7" drop if zeroing at 100 yds. So you are only correcting by 2.5 MOA or .75 mrads to get a zero. Those angles are really small so the difference on where you measure the scope height won't have any real effect on the accuracy. Except maybe at really long ranges. At 600 yds for the projectile mentioned you would need to dial up almost 16 MOA. So you will be reducing the height of the muzzle from the scope centerline by about .1 inch. That causes a change of 0.1 moa AT 600 yds which is about 5/8 of an inch. Still not enough to worry about!

That all means to me it is not an issue for you guys. Surprisingly, it is an issue in Air Rifle Field Target shooting. That is because all the projectiles are subsonic and have really bad BC. There are large corrections which need to be applied, especially at really short ranges. At 10 yards (minimum target range) the Kill Zone size is 3/8 of an inch. For a .22 projectile that means just slightly larger than the head diameter. Because of the scope height the projectile is traveling up and still needs something like a 5-7 MOA holdover to hit the KZ. It is not surprising then that new FT shooters have trouble with the short targets. The 55 yd targets (max range) have much bigger KZ's, but we are also dealing with holdovers of like 4 MOA and really have to be good estimating the effect of wind. Oh, by the way, you don't know what the range is to the target, and you can't use a range finder. Need to figure it out with your scope. In my discipline the scope is fixed to 16X and you can't use the turrets.

Again, thanks for your input