See, I actually would rather the gun be obviously fake. I can just see myself when we go camping on the beach in 2 months. I'd be standing in front of my buddy yellin, SQUEAL LIKE A PIGGY, BOY! shooting him continuously. A ranger strolling by would probably drop me if the thing looked like a real gun. The one I'm looking at is clear plastic and random bright bits of orange.
Surprisingly, the stupidest and cheapest looking air soft gun is probably the ultimate thing I am looking for. I just had no idea what kind of FPS is considered "good" or "insanely painful" or what.
Something with about 500/600FPS would be the max without killing your friend.
I don't suggest you use a pellet gun just the little plastic balls. Something with 1600FPS would be for hunting and not for friends. I actually seen a guy kill a 125LBS wild hog with one shot with his Gamo 1600FPS Air rifle. I just said to use a pellet gun on him as a joke. But please don't shoot your friend with lead pellets or BB's.