Re: Fixed 10X vs variable at 600-700 yds
Initially, 3-9x scopes were used. However, repeatability was problematic. So the 10x was introduced and became the *standard* military 'long-range' optic. Shortly thereafter the 10x optics were produced, 16x and 20x were added for the spec-ops guys. So yea, 10x will definitely be good to 600-700 yds.
Nowadays, machining is so much better than before that POI repeatability is constant in the higher-end variable powers. So if you have a choice, a *good* variable offers greater flexibility over the fixed-power optics at no loss in accuracy/repeatability.