I think it would be useful to picture certain weapons we ar familiar with, the pp90m1 damage falls off fully at 20 (60ft) metres, and I would say 20 (60ft) metres is in medium short range. 15 (45ft) metres is short ranged, where the pp starts to drop off, and 12 metres is generally the area where shotguns start to really lose their shot strength. 12-20 (36ft-60ft) is short ranged, 0-12 (36ft) is CQC.
Another weapon, the type 95, starts to lose it's 2 shot capability at 24 metres (72ft), and loses all it's strength, falling to a 5 shot kill at about 37 (111ft) m, so I would argue that 20-37 (60ft-111ft) is about medium range. Medium long would go to about 60 m (180ft) if we need that classification, assault rifles start to lose out to LMGs and Snipers after medium range, and LMGs lose out to snipers probably past 60 metres (180ft) or so.
CQC: 0-12m (36ft)
Medium: 20-37m (60ft-111ft)
Medium long: 37-60m
Long: <60m (180ft)
So, anything below 37m (111ft) for a sniper would be considered short range for that gun, anything below 20 m (60ft) would be short ranged for the AR, anything outside the 12 m (35ft) zone for shotties is iffy, anything inside 20m (60ft) for SMGs is all good because let's face it shotguns are a joke, and anything outside 20m (60ft) for an smg is long ranged for it.
There are of course discrepancies withing categories, UMP has a little more range, the Scar-L and M4A1 have a steeper damage drop off and the drop off happens sooner for these guns, The Mk-14 can go out to 60m (180ft) no problem but do have troubles inside say 28m (84ft). And the LMGs can actually perform well within say the 15-20m (45ft-60ft) zone compared to ARs because of a superior fire rate to assault rifles, although they do work best 30m (90ft) plus. You mentioned pistols, the weak powered ones full off completely to base at around 12m, so these are CQC only, the more powerful ones I tihnk go to 20 m before falling to base, but I really would not use them pas 12m because of the visual kick + recoil associated with those particular guns.
I think this is the explanation you were looking for.