How much range does 'Range' add?

Solution 1:

According to the Call of Duty wikia:

Range proficiency is only available on submachine guns and shotguns. It increases the effectiveness of the range of a player's weapon by 25% (it increases damage drop distance by 25% for the weapon; for shotguns it also increases maximum range by 25%).

The example they give using the USAS-12 shotgun is:

USAS 12's damage begins to drop at 10.5 meters and its maximum distance is nearly 17.7 meters. With Range proficiency its damage begins to drop at 13.1 meters and the maximum distance of this shotgun is increased to 22.2 meters.

Solution 2:

Until someone hacks the exact number stats from the game code like they have done with previous CoD games, we won't know for sure - hard facts have not been released anywhere. However, everything I've seen indicates it is the same as the silencer removes.