I was out checking zero on a couple of rifles this Saturday.
.243 Sako 75 with S&B 6x42. Changed ammo from 100g Norma SP to Federal Power Shock 100 g SP. Interestingly the cold bore shot after cleaning with the Federal went bullseye, great!, Norma and Fed shoot the same...NOT SO!!! next three rounds fell 2.5" low, but 'in line' (L/R wise). Raised 3 clicks, and bang, group was touching each others holes about 1/4" high and 1/4" right of center (prob my fault) at 125m, so left it be,,good enough for one last trip with the 100grainers for reds this weekend (and is prob spot on at 150m's), then bin them for good (and replace with 87g's) and await .308 this spring
Then checked zero with 6.5x55 Tikka and Norma Vulcan's, using 4-12x56 Zeiss diavari. No change in ammo, but shot 2.5" low as well, WTF! last time it was spot on. Two rounds later, a bit of adjustment, and it was spot on at 100m. One round went 4" left, but realised the barrel was touching the stock on the right side suddenly, and a bit of pushing the stock around, got the barrel working fine again must have been that trip in the highlands getting drenched for 8hrs straight
Anyway, to the point...I thought my S&B was good, but the clarity and finess in the Zeiss crosshairs allowed me to see the target boxes SOOOO much better and clearer (at 6x as well)...it was quite literally a WORLD of difference, now I'm no longer that pleased with my S&B....