LOL, He is just being a smart ass by suggesting that 5x50 op! Anyway the 3.5 is larger because it has to be in order for it to have the large field of view that it does. It's size is the trade off... And the 3.5 is exactly what I will be buying as my next optic for a 16" 1/7 H-bar. But yes I think it would physically be a little to large on 12.5" barreled gun. On that rig I would go with the 3x30 TA33 series for the ranges you want to shoot. Which begs asking, why are you wanting to shoot you're 12.5" gun out to 5 and 600 meters? It just seems counter intuitive. If it was my gun I would get the 2x20 ACOG and have an awesome little stubby rig for any task under 300 meters. But thats just me.