1J is approximately equal to 328fps using 0.2g BBs. It is a more accurate measurement than feet per second because, obviously, if a heavier BB is fired at the same feet per second, the impact energy will be greater as F=MA.
Metal BBs CAN be fired from most professional airsoft guns and by that I mean non-LPEG rifles such as TM, CA and ICS brands. Most people here don't use them as they easily exceed the 1J limit at any decent velocity, making them as useful as a chocolate teapot. Some donuts try to use paintball BBs, which are a great idea, except that the BBs often burst in the chamber, causing massive amounts of messy damage.