An Ion gun would simply be a kinetic weapon like conventional guns, but using ions (electrically charged atoms with either added or subtracted electrons/protons) .
Yeah but unless that ions in a heavy shell to accelerate the shell faster than you would with explosives, it would be kind of useless since the distance you would shot with would be ... I guess measurable in nanometers.
Where as a plasma is a material that has become electrically charged (like an ion) through a ton of HEAT (becoming a soup of uncombined electrons and protons caused by said heat, which recombine when the plasma cools down to a gas) And so the majority of the damage would come from heat (8000Kish). Think of it like a flamethrower that shoots liquid lightning (is that an accurate analogy?). I cannot find nitrogen's plasma temperature (although I found its ionization temperature is less then hydrogen's) but is used in some kind of process called "plasma spraying"?
Then it has an issue: it shots, as a flame thrower (Especially if you assume that's a flamethrower using Gas --'), only a few meters away ... :p
I'm just bothering you, I know it's just cause it sounds cool