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