DarthBoolean

Well, I can't speak for the Nerf aspects of modding the mp5, but I can definitely recommend that you paint the gun a different color than black. I used to be into Airsoft before heavy legislation in my state (Texas) made it difficult to play. Now U.S. law dictates that you must have 1/4 of an inch of the tip spray painted neon orange if the gun resembles a true firearm. Nerf guns get around this by having truly outlandish designs and already colorful paint schemes as we all know, airsoft guns are slightly different, as they are designed to resemble actual firearms. I can tell you from experience, however, that the orange tip is not visible from far away, and most police officers will overreact to you having the gun.
(Warning, long story here) This is where Airsoft got in trouble in Texas. Several kids would buy guns from the local Wal-mart or Galyans and run around outside, the police would stop, panic and draw their weapons. The kids who were young, stupid, and full of adrenaline, some of them didn't comply with the officer as fast as he/she would have liked, and some were so stupid as to even shoot the officer. (End long story)

Long story short, even if the gun will look stupid, I would go ahead and paint it just in case. If a cop decides that the gun is a threat, and pulls a gun on you, its your life or his in his mind, and I doubt he'll hesitate to pull the trigger.

And why are you worried about looking stupid? I'm assuming you'll only be using it to Nerf with other Nerfers, a community of people who play with guns meant for children, or homemade guns fashioned out of pvc pipe.