Ok then, I will accept that the xenos are immortal creatures that cannot be harmed unless you have a pulse rifle or smartgun or fire a shotgun in their mouth....
Basically thats your logic.
and has a really small clip size, like 3 or 6 bullets (well, technically it's a revolver by the name, so it should hold 6).
Which means it have big rounds. There are .22 revolvers with drum capable of holding 9 or 12 bullets but if the revolver is using .45 it will probably have just 6 holes. So having 3 or 6 bullets tops, means that revolver packs a punch.
But its not a big deal. I just don't believe that the xenos can not be killed. If its not breaking certain parts of the game making it impossible to beat, somebody will make a mod that enables the killing of the xeno