Been noticing changes in range finding esp in hot weather. Never really occurred to me that there is such a phenomenon until I read an article in hardairmag a few months back. When the gun would suddenly shoot high or low, my initial reaction was to blame the gun or the scope not holding true. Temperature was the least of my suspect.
Decided to wake up early one day when the temp was in the low 60s and the day would heat up to high 90s. I'm lucky to have targets in 5 yard increments to 55 yards.
I think I marked my scope wheel back last fall when it was colder (not sure at what temp though). Started to range find the 55 yard mark before the sun was fully out with temp at the low 60s. It was spot on. I then continued to range find as the day got warmer. At bet 68 and 70F, the shift occurred. The scope shifted about 5 yards more. The 55 yard was now focusing sharp at about 60 yards. It stayed this way to the high 90s.
Tried to google topics on scope shift but not too many I can find. But in the last video posted by AEAC titled "Pyramid Air Cup the Movie", an interview with the WFTF champion Jack Harris was a revelation. He has 3 sets of scope wheel marking for 3 specific temperature ranges (48F and below, 48F to 77F and 77F above). The interview is about 1 hour 10minutes into the video.
I use the inexpensive Tac Vector 10-40 scope. For temp reading I use an aquarium stick on thermometer.
From the video interview, it seems a digital thermometer is the way to go for a more accurate reading.
So if you have observed temperature scope shift, maybe share your experience and how you resolved it to shoot better.
My game plan now is first get a digital thermometer. Then observe at exactly temps the shift would occur with the digital reading. Will there be one or 2 shift points ? Then verify if I have to make a different holdover cheat sheet for the different shift points. Plan on shooting a pellet drop profile at 5 yard increments.
Does it sound like a sensible game plan ?
Travelbike:
First we need to differentiate scope shifts with velocity shifts...Temperature changes the velocity of your pellets, cold air is denser and your velocities will be lower, warmer-hot air has less density and your velocities will be higher, temperature also affects the air pressure in your gun and this is reflected also in the velocity...Elevation is also a factor, higher elevations means less dense air implying higher velocities.
I mention this because many shooters tend to confuse a shift in the POI and blame it on the scope while in reality it is derived from the change in velocity...Now, if your velocity or density of the air has not changed and your POI has shifted, you then have a problem with the turrets of your scope, or your range finding was deficient and your holdover was wrong. Very seldom you will find that the problem is derived from the optics which is also possible but not is high grade scopes.
Now, if temperature affected your range finding by using the parallax wheel to the side of the scope, you should understand that ALL scopes (some more than others) will have a minor shift in the range finding scale...The reasons for these are mainly optical illusions outside the scope and the discrepancy of the different thermal expansion/contraction coefficients of the metals and materials used in the range finding mechanisms: Brass, SS, Aluminum, Titanium, Plastic/synthetics, etc. all contract and expand at a different rate. Optical illusions outside the scope refer mainly to mirage even thou you don't see the waves it will be there... Heat waves exist even at close range and they tend to make us believe that objects are farther (naked eye and though the scope optics)...So if there is a shift is in the parallax you can blame it on the optics outside the scope and the thermal coefficients of the materials used in the mechanism...
Keep an eye and record at what temperature you marked your parallax wheel and verify the temperature when you noticed the shift...Good scopes should never shift if the temperatures were the same (when you marked distances and when you shot) but I have seen exceptions...One of them was a very high end Schmidt and Bender 10-50 x 60 (30 mm) that was sold as the FT scope...I bought one of these that sold for around $3,500 -3,700 US back in 2014 or so...This scope was a beauty and came equipped with all the bells and whistles but ended up being a lemon and an abomination compared to the Exceptional S&B PMII with the same power range...This FT scope shifted for any reason and for no reason and the company opted for discontinuing this model...Recently S&B launched their new version of the same scope with a 34 mm tube and it seems they corrected the problem....I have 2 S&B PMII's on my 2 impacts: A .22 carries a 5-25X and the .30 a 10-50X and both are exceptional scopes. The "PMII" models are designed for police and military use and you don't play with these markets...Exceptional repeatability, no shifts with temp changes and outstanding optics characterize S&B PM II's.
Lastly, if the shift is derived from the turrets of your scope, you might as well use your WARRANTY and have the factory fix the problem, no high grade scope should shift with temp changes (within normal parameters of course).
Best regards,
AZ
