It's the natural evolution.
Water Guns. Nerf guns. Cap guns. BB guns.
Cowboys and Indians. Cops and Robbers. Playing Army. That sort of shit.
That and I helped a friend move once and stumbled onto a G36c... Spent most of the evening after the move going room to room like a nerd.
"War must be, while we defend our lives against a destroyer who would devour all; but I do not love the bright sword for its sharpness, nor the arrow for its swiftness, nor the warrior for his glory. I love only that which they defend: the city of the Men of Numenor, and I would have her loved for her memory, her ancientry, her beauty, and her present wisdom. Not feared, save as men may fear the dignity of a man, old and wise." - J.R.R. Tolkien