A few things I noticed about the ZU-23 in ARMA 2 that are not realistic.
First: The gun barrels don't move back and forth while firing
See here:
Second: The guns are way too accurate, IRL there dispersion is much lager, this makes the guns in game too powerful against ground targets
See here:
Third: The ZU-23 guns have way too large magazine ingame, IRL the guns have only 60 rounds per barrel in a full belt, so the realistic number before changing magazines would be 120 rounds
See here: http://www.mil.fi/maavoimat/kalustoesittely/00124_en.dsp
also it may be a bug, but now the magazine change is instantaneous