So I tried the Pickup truck with the machine-gun mounted on it and was very disappointed to find that it had no reticle you can aim, you pretty much have to guess where it is going to hit and aim accordingly by trail and error which can be very frustrating.

I suggest giving it some Iron-sights at least to make it more realistic..

I can't change 3d assets, its just a vehicle from ARMA 2 it's not something I can make changes to in a mission file. Many mounted guns like the one in a huey will have an iron sights mode if you press numpad 0, not sure if you tried that.

Didn't try that one, but in case of gun truck second mouse buton should work. But it's bugging somehow (it's not Dayz problem, I have that problem in ARMA editor aswell), and it's hard to force it to work (can't even explain how to do it because I'm trying everything - bailing in and out, relogging and so).