From my experience not a lot of people have thought of the combination, but any gun that has a way to stack damage/dps infinitely (Deagle being my preferred option) can take caliber reduction for essentially free stats. Also useful on some guns that have a very high amount of damage that usually overkills, but are unwieldy.
Other weapons worth taking it on include;
P9000
Tactical Observer
Laser Socom
RRX Coil Shotgun
Sudden DMR
The thing is, in S1 damage is a stat that comes fairly easily and can be augmented/supplemented by a very wide array of items. Accuracy and mag size are considerably harder to come by. Caliber reduction is a very strong pick on basically everything.