Place a Trigger on the weapon spot ya wanna create.
Then NAME the trigger in the "name" field - let's name it G1
and just let it fire right away (by anyone).
Put this in the "OnActivation" field :
W1="WeaponHolder" createVehicle Getpos G1; W1 addMagazineCargo ["M16",4]; W1 addWeaponCargo ["M16",1]
This would place a fully loaded M16 on the weapon spot (the "G1"-trigger, right?!)
NOTE :
Sometimes (when placing handguns at least), it can take a while before yer able to actually pick up the gun.
Another thing (again, with handguns at least), you sometimes have to click on the "toggle Weapons" button
to be able to shoot (it makes the mags visible, top left of screen).