I've done a lot of work looking into scopes and temp and found the following. You have to assess what people's experiences are of shooting when talking about shift. Have they been exposed to a variety of conditions, and are they able to determine the difference between scope shift, mechanical shift of the gun, and enviromental shift? Is it the shooter? What is their competition experience like? Put it this way, i've been looking at it for about 7 years and i am only just confident enough in knowledge to say there's 3 main types.
All scopes move, but some move a lot less, and/or at higher/lower temperatures. Scopes of the same make and spec can vary in their behaviour (more of that in a sec). Some snap change like a switch, some gradually shift. Many are convinced it's light conditions, however i've been able to get my scope to move just by heating it gently in the sun.
Silver and white scopes reduce the speed in which a scope can shift, but given the same exposure to ambient temp, they will catch up. The best protection for direct heat like sunlight is a white towel. Keeping it a few inches off the ground on a bag, or in grass on a hot day also can keep it lower. Putting it in a bag can help, but a black bag can make it worse. Sticking it on the ground in winter can lower it a lot.
You need to learn your scope, where it shifts and how much by. Cheap temp strips used in aquariums and brewing can give you an indicator of what it's doing. I've got them on ever scope, going from sub zero to + 40 ( i think)
I know my march goes shifts 2.5mm around the wheel at 26-28 degrees and another 1.5 mm at 29 degrees and stays there. That equates to 4yds at the top end, but naff all below 25yds. My debens and custom shops don't move unless they get really hot... the only time i've seen them move was in SA. But it's taken me a year to learn. The first year i had it there were only 2 shots that affected it (both at the same venue). This year there's been 2 already.
I think it might move at -2 but my eyes are problematic at the moment so the jury is out on that.
You need to check scope shift without firing a shot, just on known targets, in the same light conditions, ranging on the same thing. The amount of light on a target can change how your eye reacts, and hot and cold days can affect trajectory as well, considerably. I use string and newspaper, never ever the plate unless it's the last resort. Keep everything consistent but the scope temp. If you want to heat a scope up, stick it in the car for 20 mins on a hot day, you can even turn the heating or ac on to vary that more.
Scopes like the custom shops, nikkos are pretty temp stable and in the same price range. Leups comps are a mixed bag, i know an owner who says his doesn't shift, or he doesn't notice any, who is a top AA shooter, yet another top AA says it goes a long way out. Both have won the GP series and know what they are doing. I think this perhaps is down to a change in manufacturing over the production life. 20-50's i've heard switch. My theory is that scopes are made in a slightly different way or using different materials.
The 34mm tubed scopes seem to be shifting at higher temps. My 30mm march flicks (as above) almost like a switch, and the fated S&B FT was terrible by most accounts (enough for a petition to be raised at the worlds in Hungary!) It wandered off at any known temp, a degree a yard perhaps!. The S&B PMII however seems quite solid, with reports of a little shift at higher temps.
It may be the more expensive scopes are using a more expensive material, are less expansion tolerant, or there is a different design internally between those that flick like a switch, those that are gradual, and those that appear not to budge. But you need to learn you scope.
I'm getting to the end of learning my march, but i don't jest when i say i've been tempted to throw my deben on now and then, which i know doesn't move enough to be upset by UK temps.
So, if you like your scope, the chances are it's a learning thing. There may be things you can do to keep it away from higher temps, like covering it, but there may be days when it's hard to avoid it. If the ambient gets high (like it did at the weekend), the scope will gradually get there. If it's just a bright but cold day, then keeping it in the shade can keep it down. I've seen 15 degrees in the sun in winter when the ambient is just 5. On a warm day, with 18 in the shade, i've seen 36 in the sun on baked mud.
PS at the weekend my trajectory shifted as well. I never dialed more than my 47 yd setting. But knowing my scope to a certain degree made me far more confident of what was going on.
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