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the reason why we call them markers is cuz when you call them guns the government get confused by it alot and we trying to stay away from calling them guns so they can let us use them to play and own them so we call them markers now cuz they leave paint marks and when you call them markers it is easier for people to know what they are instead of thinking you own a real gun.    

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"Many paintball aficionados claim that the proper term is “paintball marker”. Their logic is simple - the point of the game is to “tag” or “mark” your opponent. Really, though, I’m pretty sure that their focus on calling it a paintball marker comes form their desire to distance the sport from violent, extremist, survivalist impressions that many people still have of the sport. “If we call them markers, then we won’t give the impression that we’re hunting other people.” Some even claim that September 11th gave the impetus to really adopt the name and try to separate the sport of paintball from any sort of radical movement.

Those that prefer to call them paintball guns, though, have their own reasons. Their best argument is simply to call it what it is. A gun is a device that shoots projectiles and is actuated by a trigger. A paintball gun fits that billing. While its definitely not a firearm, it’s just as much a gun as a BB gun or squirt gun and there doesn’t seem to be any great push to start calling a squirt gun a “water sprayer”. "

http://paintball.about.com/od/howtoplay/f/marker_or_gun.htm