A sniper does not necessarily mean a gun itself. A sniper is basically a highly trained marksman. The proper word for the gun is a sniper rifle. Which is a very powerful rifle with a scope designed for snipers.
Noob: I'm going to buy that sniper. (he's gonna buy the person)
Other dude: No don't you mean a sniper rifle?
Noob: Shut up noob.
Other dude: ...