How Effective is the MK12?

[BigLarge]

Not really on topic...

but what caliber would you suggest the military use that would be silent, and capable, at 600+ meters?

The super-simple answer would be to run 7.62NATO, and that's often what gets used when magazine/ammunition inter-compatibility isn't the requirement, since it's great at that in a precision system - same concept for a subsonic load that can be used for quiet one-off use even if it's non-ideal. It needn't be super-quiet (first round pop + sound of the action cycling is going to be fairly loud) to be an improvement, enough signature mitigation to make location identification of the shooter hard is helpful, and with really good optics again (3-18x or 5-25x from Leupold or S&B respectively) they can do more than just put rounds on target for COIN ops.

I was being sarcastic towards the other guy. Making the statement "a suppressed 556 would not be my choice of weapon to silently do anything" is off-topic and frankly offers no meaningful information. My point is, there is no round available (that i'm aware of) that allows you to quietly and effectively engage targets at 600+ meters.