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.
Same, it's good even on weapons that don't have any damage stacking ability as you can just pump extra upgrade kits into it to fix the damage output (even better if the gun is modular)
Though since the P9000 and the SOCOM progressively get more accurate anyway as you enter each room, I don't prioritize accuracy attachments on either of them since they will eventually reach 0 deviation and 0 recoil anyway. It's still worth taking for the magsize increase though.
i would take it on basically every weapon, especially if im looping. the damage decrease is fairly negligable, and better mag and accuracy is always good