You probably could have asked the question in your last thread instead of wasting space by making a new one.
If you looked at the tutorial I gave you the rigging process surely would have become clear and show exactly what it is...
But to answer you question - generally you wouldn't rig a gun - though big guns to reload like a shotgun...(Well I suppose you essentially could 'rig' a trigger if you want to call it that)
Rigging is basically the process setting up so called 'bones' each of these 'bones' will control a set of point or 'vertices' when you rotate these bones in animation these vertices will follow the bones relative to it's angle and position - much like certain bones control parts of out body - though the use of a bone on the trigger object may make the animation smoother.
Hakuna Matata