Like most everyone else here I grew up playing with toy guns playing "War", "Cops and Robbers", "Cowboys and Indians", "Batman", "Star Trek" and the like. There were anywhere from 12-20 boys aged 5-15 in the neighborhood at any given time. (we also played a bunch of Baseball, Football, Basketball etc.) we ran around and terrorized the neighborhood in a pack of kids running around screaming their fool heads off kinda way.
Just about every house also had real guns in them.
And we knew that while setting the back yard on fire will get your ass beat, touching a real gun would get you killed, one way or another. And we never did touch a real gun. Ever. Until we were old enough to be taken hunting or shooting.
Now my son is 10 and has toy guns, Nerf Guns, Super Soakers and Cap Guns (do you have any idea how hard it is to find real caps and cap guns these days?!?!?!) And he goes hunting and shooting with me. There is no doubt in either my mind or his about the difference between toy guns and real guns.
In fact the only problem has been 1) Yes, you can shoot your sister with your Nerf Gun, but you just better be able to outrun the beatdown she'll give you if you do. 2) Don't shoot the dog. Seriously, I mean it. Don't shoot the dog. If he bites you. I'll laugh, because you deserve it.
He hasn't shot the dog. However, there has been a time or two when I heard my daughter scream my son's name, him hauling ass through the dining room and out the garage door with a Nerf Gun in one hand and a big o'l cheese-eating grin on his face, followed shortly thereafter by my extremely pissed off teenage daughter in hot pursuit.