When you aim directly up in the prone or urban prone position, or at least 45 degrees up, you look, at best, derpy.
This would require a lot of flexibility and abdomen strength in real life, plus adding the 40lbs - 150lbs you have to carry. Combining all of these factors with make it definitely impossible, and it's looks just plain derpy.
I would request to put a limit to how high you can aim while prone and urban prone, and if you aim any higher than that angle, your gun stops moving but you head continues to look up (similar to looking around using the ALT key or TrackIR).
An alternative and BETTER solution is when you reach a certain aiming height, you turn over and roll to your back. It would be a much better animation than going to high prone, because your silhouette is still much lower than high prone.
Remember, this game is a military SIMULATOR. It needs to be as realistic as possible. {F20329} {F20330}