My question: Which fixed 6X did the USAMU recommend?
From the USAMU Counter Sniper Guide:
The list few scopes, mounts and rifles giving the order of preference.
Scope: Redfield 6X with 4 post cross hairs
Mounts: Redfield single piece
Rifle: Remington 700 Varmit Special
Understand this was written in the mid 70s but it worked then and it will work now.
Range to the target could easily be miscalculated if the scope were a zoom scope and not set at the proper magnification for ranging.
This is true if your scope is calibrated at the second focus plain. It's then valid at the highest magnification. But if the scope is calibrated at the first focal plain, its valid through out the range.
Meaning your mil dot on the second focal plain works only at the highest magnification where as the mil dots will work at any setting when the scope is calibrated at the first focal plain.