I'm not expert in the sport of shooting, but if the world record is calculated by the amount of points, how can you beat a perfect score? You can see the records on the official site for the London Olympics, and there is no mentioning of time. So can a score of 600 still be beaten? And what's with the 704.8?
In the event you mention (50m Rifle Prone), the 8 best qualifiers shoot in the final event. In the final event, the scoring zones are divided into tenths, with 10.9 being the maximum score per shot (ten shots are taken).
The final total is the combination of the qualification and final score.
So, if you shoot in the final, you could hit a perfect 600 (in qualification) and then your final ten could be anything up to 109, meaning a maximum possible of 709.
You note the 704.8 but actually, in the London 2012 games just now, Sergei Martynov won gold with 705.5.
I'll add also that, once someone shoots 709, that will be the maximum world record (and it will be shared by anyone else shooting 709) unless the scoring changes.
For clarity, this is how the shooting is scored:
Final (Top 8 qualifying shooters)