If you are wanting to size a moose’s antlers then I’d suggest a ranging reticle, a range finder, tripod or bipod, calculator, formulas and patience. If you use MOA then 1 MOA at 100 yards is equal to 1.047”. If you use MIL then it’s 3.6” or 10cm. With the formula you can plug in distance and how many MIL or MOA the target is wide and figure out its size. With that knowledge I know that 48 MOA at 100 yards is 50.25”. The bull I took last year was 50.5”. Patience, a stable platform, good reticle and rangefinder gave me the data I needed to shoot the bull I did. I don’t have any pictures of him via spotting scope but here’s one on the ground.
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His right side is almost 50% larger than his left side (when looking at it head on). That also serves to mess with your perception when viewing them. I thought he was much larger from up on top of the mountain but when I got to him I was able to see the size difference. Once measured though, he went down.