Yeah if you get a full metal one it gets pretty close to the actual weight of the real thing. Ive held one before and its pretty damn heavy but I'd have to say the funniest one to try and shoot standing up is the sniper rifle the Canadian army uses (I forgot the name). I held that thing... Honestly you have to throw your hip out to support your arm to shoot it. I couldnt do it, but the snipers could, so thats all that matters ;).

do these m203 launch a projectile that fires bb's out or does it simply fire a large burst of bbs straight from the barrel ?

It shoots a burst of bbs...

Using grenades that hold the bbs... propelled by a type of gas (usually propane).