A scope tube made of 6061T6 aluminium will have a linear thermal coefficient of expansion of 23.4 ppm/DegC, which is the same as saying 0.00234 %/DegC.
So a scope that is 300mm long will expand by 0.155mm in a change of 40 F or 22.2 DegC (going from 4 Degc to 26 Degc, say).
This is aircraft aluminium, so for comparison, a Boeing 747 that's 231 feet long would expand 1.43 inches. This would be 12.9 inches if the math quoted above were correct, I think.
