400 FPS is the limit on many fields for what is considered safe with AEGs (ie capable of full auto). For CQB the limits in Quebec are usually around 350 FPS.
Minimum engagement distances are usually set to around 15 feet.
As for injuries, there's harm and then there's harm. I've gotten broken skin from being shot by a 250 FPS gun from about 10 feet away. If you're worried about small injuries like that then you really should wear full protective gear, or perhaps consider a different hobby (like Bingo :P ).
Otherwise if you're wearing proper eye protection and a mouth guard, 400 FPS won't cause much more harm than small welts and occasional broken skin.