As a career move, Nerf making real guns might not be such a good idea if they're going to use the same style and design as their foam dart guns to create a "revolutionized" firearm. As an ethical move, wouldn't Nerf be teaching little kids that guns are good, buy these guns and shoot your friends, stuff like that?
Also, in my opinion, I think Nerf (at least in their newer machine gun models) take the design of actual guns and modify their looks to make them more fun or less serious, so much so that it looks appealing to parents, as well as the children that want to play with them.
But, the reality of the situation is, Nerf will probably never even make it to the "Big Gun" manufacturers because they started out as a toy company, and will probably stay the way it is until the consumers deem it boring or dangerous etc.