Ehm The 50 cal already is. There are reasons I suppose. Don't ask me to think of one, just pointing out that the little knob on the back end holds down the button to make it full auto
http://www.m2gun.com/images/fig3-2.gif
Misread. You got it. But anyway, yeah, I can't really fathom a reason why I need to single fire a 50...