I now own several Leupold VXIII LR scopes one a 4.5 x14x50 with gen 2 reticle from premeire reticle with out target knobs and a 6.5X20X50 with the varmint hunter reticle and target knobs with dust covers(pain in the butt). I purchased the 6.5 for a long range rifle I had built last year. Does anyone know the mechanical difference between these two models of the same power? I was thinking of sending the 6.5 in to upgrade the turrets to the M4. Is that worth the money or would just purchasing the Mark 4 be a better way to go?