A friend of mine who trained me in precision rifle shooting (Brent Clearman
http://www.camemorial.org/2001-present/clearman.php) told me about a couple of times Marines he worked with shot melons out of each others arms at 100m as a right of passage.....I always wondered if he was just giving me a hard time, but after some of the crazy stuff we did together, I started to believe him.
But shooting one off your head with that rifle and ammunition (it looked like a surplus round to me, I could be wrong, but I doubt it)....the shooter did numerous things incorrectly as well...I don't know, if it wasn't faked, then they are completely insane! The close up camera on the guy had a definite large caliber sound signature if you ask me, but that coule be easily put in with modern sound/video editing software, so it's just as likely they used a firework of some kind to blow the melon and did a sweet edit job...also the background behind the target, there should have been an visual impact and there wasn't (.50's kick up a lot of dust/earth/etc)....either way, awesome video, haha.
EDIT: After watching some of this guys other videos, he is obviously tricked right out to do special effects and such, so I'd bet it's a fake, but an exceptionally well done one.