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Optical zero? |
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kruzaroad 
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Joined: 03 Jul 2022 Location: Hastings 4 now Status: Offline Posts: 2635 |
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 Topic: Optical zero?Posted: 24 Sep 2025 at 1:25pm |
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So I've been aware for a while of the technique of tin foil over scope lens with a small hole, set zoom lowest,ao lowest,get target as close as possible within foucus and fire a shot then measure from bulls to shot for scope height.
Supposed to give more accurate measurements in chair gun. So finally going to try it. The one thing i cant find on any video is wether I need to be set to optical zero, or if i can do it sighted in. Can anyone help me on this please. Cheers |
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RangerPete
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Joined: 19 Apr 2023 Location: Cambridge. Status: Offline Posts: 1028 |
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Posted: 24 Sep 2025 at 4:51pm |
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Hi Kruza, what is it that you are trying to work out, the scope height?
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Walk quietly, but carry a big stick.
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kruzaroad
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Posted: 24 Sep 2025 at 5:15pm |
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Hi Pete.
Yep but Im trying this way to see if it's more accurate than the half scope height and center of bore. |
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Billbobnz
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Posted: 24 Sep 2025 at 8:48pm |
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This may be helpful
https://airgunforums.co.uk/threads/chairgun-tutorial.26534/ I will try and find that video of the tinfoil method |
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Billbobnz
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Posted: 24 Sep 2025 at 8:53pm |
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This is in the above which I thought was interesting.
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nunga
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Posted: 24 Sep 2025 at 11:38pm |
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Not sure how that method will give a more accurate measurement because you are limited to a number of factors. first off you have to make sure cross hairs in the scope are bang on center and that is floored because the cross hairs move up and down inside the scope which will give you a false height unless you are sure that is also dead center. Then you have to work out the center of the pellet hole in the paper target you have just shot. Too many things can go wrong. I have always done it the following way. Outside diameter of the barrel % by 2 (if the scope is long enough) or measure the outside diameter of the breach and divide that by two. and then the outside diameter of the scope and % by 2. Add the gap between the scope and the barrel plus the two other numbers and you cant go wrong. i use my Mitutoyo vernier calipers for this. To me this is a better method because the scope is fixed to the gun and that will not change
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2nd place HP springer - WFTF World Championships 2014
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kruzaroad
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Posted: 25 Sep 2025 at 8:23am |
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The scope is mounted to the rifle this way too!
The rifle fore end can be held in hand and bag rested , and the wrist and trigger hand steadied by a towel or such. Distance is about 6m and im going to do it in the shipping container for no wind. The bag and towel technique is incredibly steady and my standard set up for distance sighting. I had a yak with chat gpt and it gave me the equation to work out scope height from my rifle as set to 50m. Here it requirements is for anyone interested. Pin hole in tinfoil in mm X= muzzle to target in meters Zero distance (optional) Average shots If have wedges in. Scope model I have the equation but can't do the division equation set up on this ph, so if anyone wants it let me know and i photo it. Personally I'll just use chat gpt to do it. Honestly I'll be checking how close it is to my more traditionally measured scope height first. I'm experimenting Nunga. Seeing if something different works better. Im doubtful as it doesn't seem to require a lot of info but its all angles and maths and if there is one thing chat gpt beats us all at is mathematical logic. Time will tell ( i will too after it) |
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kruzaroad
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Posted: 25 Sep 2025 at 1:19pm |
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Well that was crap.
Gave it wealth of info and it came back with 17.1mm Though it did have some good info internal scope measurements. Wants the thickness of the barrel so can work out height now. So much for angle calculations. 15mm high at 6.26m |
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nunga
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Posted: 25 Sep 2025 at 5:29pm |
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One thing to take into account as well is that the pellet does not travel out of your barrel in a straight line. If your zero is set for say 30 meters and you put your tin foil over the lens and move the target as close as possible your pellet will be hitting very low. Which will give you wrong data to work off.
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2nd place HP springer - WFTF World Championships 2014
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Pauly5
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Posted: 26 Sep 2025 at 8:32am |
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So the hardest thing is getting your scope to focus so close, ideally at point blank. When I have measured my scope height I just use callipers, but try and verify it by shooting at something as close as possible. The digital scopes do quite well in focusing to a couple of metres, so last time I did this, I shot at a 2 m target and from memory was only around a mm difference which was good enough to verify I had measured correctly.
Doesn't help you Kruz, but if you can somehow focus your scope on a point at point blank range, you will get your scope height.
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nunga
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Posted: 26 Sep 2025 at 10:04am |
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Paul, That is what the hole in the tin foil is for. it changes the F ratio of the scope that is an old trick that they did with reflector scopes (telescopes). The biggest problem is the cross hairs and the design of the internals of your scope. There are glass optics that move up and down left to right that are inside the scope tube. but the cross hairs always look as if they are in the center even if you have the turret turned right to the top or to the bottom so you will not know for sure if you are truly looking at the center or not.
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2nd place HP springer - WFTF World Championships 2014
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kruzaroad
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Posted: 26 Sep 2025 at 3:24pm |
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Thats what I was expecting gpt to do.
I gave it the pellet weight and volicity, shim angle, zero, scope, rifle, distance, how high above bull it hit. My thinking was it would work out arc of pellet of pellet for that distance, 50m. Then then check at 6.25m that pellet was 15mm above bull,.work out how many clicks above zero i was, subtract the 25• shim angle out from the equation, give me a scope height. Mathematical it sounds feasible to me. |
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