Chairgun scope height

[Elo76]

Was using the program with the standard 1.75 for scope height.  All my numbers seemed to work out ok going from 24yrds to 10yrds with the mildots.  Today i actually measured the distance and got 2.38 which completely messed with my numbers.  Im shooting a D34 with a utg drooper mount.  What could it be?  Maybe im missing something here.

[aceflier]

Measure from center of barrel to center of scope or crosshair. The droop mount could mess with it also?

[Elo76]

I measured the tube and then the barrel.  Divided that by 2 and added that number to the measurement i got from the top of the barrel to the bottom of the scope tube.  Does that sound right?

[rsterne]

Yep, sounds like you did the calculation correctly.... However if the barrel has droop, that reduces the distance you must use in Chairgun.... because the bore line is closer to the scope since it angles upwards behind the breech.... Also, if your pellet BC is not correct, or the velocity wrong.... or if your MilDots are calibrated at some magnification other than what you are using.... any of those can mess up your answer as well....

Bob

[BenjiHunter]

I measured the tube and then the barrel.  Divided that by 2 and added that number to the measurement i got from the top of the barrel to the bottom of the scope tube.  Does that sound right?

You measure from the top of the scope tube to the bottom of the barrel.
Take that distance and subtract half the scope tube dia and half the barrel dia, and you will get center to center distance.

[Elo76]

I guess im measuring from the bottom of the 30mm scope tube to the top of the compression tube.  Not the actual barrel.  Is this correct?  Not sure how i would measure to the barrel when it is ahead of the scope.

[BenjiHunter]

Yes, and then add haft the scope tube dia and half the compression tub dia.

[jumpin]

I followed the "hole in aluminum foil" method on a couple of springers last weekend. It was easy and consistent. You might search on that method as it seems to give the right dimension - center of muzzle bore to optical sight line. Good luck.

[TwiceHorn]

For whatever reason, I couldn't make the foil trick work (guess I wasn't poking a hole close enough to the center).  So I have never had an "accurate" scope height to plug in.

Regardless, you can mess around with scope height and BC (it's fairly absurd to have a static BC as it varies with velocity) until the curve fits what you shoot at a selection of near and far distances.  Still, I could never get it to match both the far (45-55 yard) and near (10-20) distances for field target, which probably indicates some barrel droop, although my scope is reasonably centered mechanically and it's an HW97K.  I understand there is a place where you can "twiddle" other data to make it work, but I gave up on it and just shot the distances.

[rsterne]

I would suggest that if the original scope height of 1.75" worked, that you use it....

Bob

[Elo76]

I was just thinking the same thing.  Im sighted in for 25 and the scope height of 1.75 got me really close at 10yrds.  I think im gonna play with that number till i get what i actually shot at 10yrds. Then verify other ranges and see.  Its gotta be because of the drooper.

[Josan]

To make the Chairgun trajectory fit your real trajectory closely i usually do the following. If you know your real life holdover and holdunder at various distances and your zero:
- enter the correct pellet weight
- enter the muzzle velocity
- enter your zero distance
- tweak the scope height to make the trajectory fit at the shorter distances
- tweak your BC to make the trajectory fit at the longer distances

[rsterne]

Jo, good advice....

Bob

[Scotchmo]

To make the Chairgun trajectory fit your real trajectory closely i usually do the following. If you know your real life holdover and holdunder at various distances and your zero:
- enter the correct pellet weight
- enter the muzzle velocity
- enter your zero distance
- tweak the scope height to make the trajectory fit at the shorter distances
- tweak your BC to make the trajectory fit at the longer distances

- enter the correct pellet weight - for trajectory, pellet weight doe not matter, only BC
- enter the muzzle velocity
- enter your zero distance
- tweak the scope height to make the trajectory fit at the shorter distances
- tweak your BC to make the trajectory fit at the longer distances

You also need to measure and compensate for your trajectory. And it must also be correct:

Are you measuring drop in inches? If so, then measure physically at the target. Measure to the center of groups.
Are you using holdover or measuring drop in mil-dots? If so then verify the dot spacing at the magnification and distance being used. Adjust in ChairGun if needed.
Are you correcting for drop by clicking? If so, then verify actual click values at the distance used. Adjust in ChairGun if needed.

[Josan]

- enter the correct pellet weight - for trajectory, pellet weight doe not matter, only BC
- enter the muzzle velocity
- enter your zero distance
- tweak the scope height to make the trajectory fit at the shorter distances
- tweak your BC to make the trajectory fit at the longer distances

You also need to measure and compensate for your trajectory. And it must also be correct:

Are you measuring drop in inches? If so, then measure physically at the target. Measure to the center of groups.
Are you using holdover or measuring drop in mil-dots? If so then verify the dot spacing at the magnification and distance being used. Adjust in ChairGun if needed.
Are you correcting for drop by clicking? If so, then verify actual click values at the distance used. Adjust in ChairGun if needed.

I was only describing how i go about getting a maximum match for my Chairgun curve, but i will try to answer your questions.

- enter the correct pellet weight - for trajectory, pellet weight doe not matter, only BC
I'm not sure about this one. In a vacuum a pellet of any weight will drop with the same speed, but in real life a pellet will shed energy and the starting energy (weight and speed) will probably matter for the outcome of the calculation by Chairgun.

- enter the muzzle velocity
- enter your zero distance
- tweak the scope height to make the trajectory fit at the shorter distances
- tweak your BC to make the trajectory fit at the longer distances

You also need to measure and compensate for your trajectory. I reconstruct my real life trajectory by manipulating the dials in Chairgun. My real life trajectory is measured by shooting at paper at various distances. And it must also be correct:

Are you measuring drop in inches? If so, then measure physically at the target. Measure to the center of groups. I measure at the target in centimeters and usually write down the mildot drop also. Note: i have calibrated my mildot reticle in advance.
Are you using holdover or measuring drop in mil-dots? If so then verify the dot spacing at the magnification and distance being used. Adjust in ChairGun if needed. See above. I calibrate my mildots by putting dots at specific distance on paper and then placing the paper at a specified distance, then adjust the magnification untill the dots on paper and the mildots coincide. I can explain this in more detail if needed.
Are you correcting for drop by clicking? If so, then verify actual click values at the distance used. Adjust in ChairGun if needed. No, i always correct by using holdover according to the mildots. I shoot HFT in competition and your not allowed to adjust your scope during a competition, according to UKAHFT rules.

[Josan]

You know what, i'll describe my way of calibrating the mildots right now. 

This methods holds for reticles that are true mildot at 10 but you want to shoot at 10x or maybe some other magnification. Most scopes, like most Hawke scopes, that have mildot reticles are true mildot at 10x.

This is a metric story by the way.

Suppose you want to shoot at 8x magnification:
- dial in 8x on the magnification ring of your scope
- put some dots on paper with a 3cm spacing
- put the paper on 8x3 = 24m distance
- look through the scope and adjust the magnification until the mildots coincide with the dots on paper.

Now your mildots are calibrated for 8x magnification.

In this example you can use other magnifications and dot spacing on paper, like:
7x and 5cm spaced dots, put paper at 35m
12x and 4cm spaced dots, put paper at 48m

Now with this calibrated reticle you can measure distance. For example if you have calibrated for 8x and you have a 15cm tin can in the field that measures 3 mildots in your scope, the distance is:
(8 x 15)/3 =40m

in general:

(8 x any size in the field in cm)/size in mildots through the scope = distance in m.

A bit off topic i'm afraid, but i hope this will help.

[Scotchmo]

...
Are you measuring drop in inches? If so, then measure physically at the target. Measure to the center of groups. I measure at the target in centimeters and usually write down the mildot drop also. Note: i have calibrated my mildot reticle in advance.
Are you using holdover or measuring drop in mil-dots? If so then verify the dot spacing at the magnification and distance being used. Adjust in ChairGun if needed. See above. I calibrate my mildots by putting dots at specific distance on paper and then placing the paper at a specified distance, then adjust the magnification untill the dots on paper and the mildots coincide. I can explain this in more detail if needed.
Are you correcting for drop by clicking? If so, then verify actual click values at the distance used. Adjust in ChairGun if needed. No, i always correct by using holdover according to the mildots. I shoot HFT in competition and your not allowed to adjust your scope during a competition, according to UKAHFT rules.

Jason,
You are doing it the right way, so doubtful you would have any problem. Sorry - those follow up questions were meant for Ed (Elo76).

FYI: pellet weight does not matter for trajectory as the BC already integrates the pellet mass.