https://farm2.staticflickr.com/1541/24963591715_0d500dbf95_o.jpg
Here is the Institute Pistol/Rifle with the stats for an attached Standard Stock/Pistol Grip. Note the Range.
https://farm2.staticflickr.com/1531/24668098060_d0c48e712d_o.jpg
Here is the Institute Pistol/Rifle moments later with the stats for an attached Full Stock/Rifle Grip. Note the Range.
The photo also shows the tooltips listing of the mods' effects, and none of them make any mention of changing the weapon's range, so it's odd enough that the range stat would be impacted. But, it's even more strange that the full stock actually reduces the range by around 30%. If anything, one would expect a small range increase (or none at all, as the tooltip says.)
The range appears, to me, to be correct with the Full Stock. The range for the weapon with a pistol grip appears to be far too high. It would have more range than any rifle in the game with that configuration.
My best guess is a bug in the calculation caused by adding/moving mods in a particular order, but I wouldn't know where to begin testing it.
Also, I should mention that I am playing a version 1.2, not 1.3, so this isn't with the most recent patch.