Scope height measurement Question

[Glockaxis]

Hello all. I'm getting a scope that comes with ballistic calculations and had a question about measuring scope height. I know how it is calculated on powder burners, but some of my airguns have a shrouded barrel (some easy to take off, others not). Should I be using the barrel itself no matter what, or the shroud measurement? I apologize if this is a stupid question, but I'm looking to be less ignorant Thanks in advance.

 

[NAProf]

Hi,
The scope height is measure from the center of the bore to the center of the objective lens. Easiest way to do it is measue the shroud diameter and take 1/2 of that. Add it to 1/2 the bell housing (around the objective lens) diameter and add both of those to the distance from the bottom of the bell housing to the top of the shroud.
Cheers,
Greg

 

[Glockaxis]

Thanks Greg, I really appreciate it!

Dom

 

[tigereye]

I use a portable cmm for this, pretty accurate. you can also see if there is a slant as well

 

[thane]

... except for the fact the erector tube pivots near the eyepiece, making center higher or lower or posibly the same as center of objective lens.
Also factor 20MOA scope mounts... not as easy a measurement as it first appears.

 

[Scotchmo]

... except for the fact the erector tube pivots near the eyepiece, making center higher or lower or posibly the same as center of objective lens.
Also factor 20MOA scope mounts... not as easy a measurement as it first appears.

That’s why I take height measurements near the magnification ring as it usually straddles the erector tube gimbal. I take my distance measurements from there as well, for a related reason.

 

[357cal]

Hello all. I'm getting a scope that comes with ballistic calculations and had a question about measuring scope height. I know how it is calculated on powder burners, but some of my airguns have a shrouded barrel (some easy to take off, others not). Should I be using the barrel itself no matter what, or the shroud measurement? I apologize if this is a stupid question, but I'm looking to be less ignorant Thanks in advance.

It's not a stupid question at all.

 

[JimD]

I also measure the OD of the scope and the Shroud and the space between them but I still end up adjusting this a bit in my ballistics program to get results consistent with the groups I've shot at 10, 15, 20, 25, 30, and 35 yards (I would go longer if I had the space). I'm not making big changes but small adjustments seem to give me a better match to my shooting results. I also adjust bc a bit sometimes.

 

[Scotchmo]

… but I still end up adjusting this a bit in my ballistics program to get results consistent with the groups I've shot at 10, 15, 20, ….

What’s your point of reference for those target distances? The muzzle? The scope objective lens? It should be at the last focal plane, which happens to be near the erector tube gimbal.

 

[JimD]

It is roughly the muzzle. I set 12x12 inch concrete pavers in my yard 10, 15, 20, 25, 30, and 35 yards from the inside edge of the railing around the side porch where my shooting bench is. I used a steel rule 100 feet long (so the 35 yard could be a bit off). The gun then sits on the shooting bench that is up against the same railing. But depending on the length of the gun the muzzle could be a few inches from the edge of the railing. I move a 4x4 post with an old brake rotor at the bottom from concrete paver to paver and the pellet trap sits on top. The rotor is big enough it has to sit in the middle of the paver.

The gimbal on the scope would be about half the gun length away. Probably 1-2 feet away. I guess that might be part of the reason that the program's predictions don't perfectly match my shooting results. But since I tweak the program until I get it aligned I don't know what difference it would make in the end.

 

[Scotchmo]

It is roughly the muzzle….

The gimbal on the scope would be about half the gun length away. Probably 1-2 feet away. I guess that might be part of the reason that the program's predictions don't perfectly match my shooting results. But since I tweak the program until I get it aligned I don't know what difference it would make in the end.

The scope does not “know” where the muzzle is. All of the holdovers and click needed to compensate for the trajectory are taken from the scope. Or the gimbal/focal plane to be more exact. If that target distance is 10.5yds rather than the 10yds that you are using. It will definitely throw off your clicks or holdover predicted by a program like Strelok. We can fudge the scope height to get clicks to line up at close distances and fudge the BC and/or velocity for some farther distances. I’ve have done both of those fudges. But I prefer to get my input data and target distances correct.

I usually don’t shoot at more than three distances to setup my gun. I use the ballistics program to get my click and holdover data for all other distances that I might see in a match, so I need accurate input. For extreme long range setups, l shoot a few more distances to verify. Fudging the wrong ballistics data to match actual impact points can mess things up when I start accounting for environmental conditions at long range.

If I am going to shoot at all the expected distances and conditions ahead of time, and fudge the input data in the program to match the poi, then do I really even need a ballistics program?