Come on, guys how hard can it be???
1) Every gun has X number of crew (usually three). You have to kill them all to silence the gun.
2) There are no stats for the gun, because you fire at the crew.
3) The crew get cover from the gun shield.
4) You can fire at the crew if you can see the gun, even if the crew hides behind the gun shield.
To me, all the above seems logical and realistic. Only omission seems to be the re-crewing issue, but that is probably a game speed/balance thing, otherwise it would take forever to knock out a gun, and a beardy tactic would be to buy lots of cheap squads to hang around near any guns. You could do a house rule if you wanted to though.