Hello, soldier here.
I have been in the army for 20 years, and spent nearly 3 years with an SF Unit. I have never used any kind of sniper rifle, but have been trained to use an SMG, SLR, SA-80 rifle, 9mm Browning Pistol, 9mm Sig Sauer 229 pistol, GPMG, MP5, HK53, and Chieftain/Challenger 120mm Main armament. Regardless of range, weapon, or SF scenario the drill when engaging ANY target is "aim for the centre of the observed mass". NEVER have I seen ANY soldier trained/training in shooting weapons from a combatant's hand, or shooting to wound. NO 'sharpshooter' in the WORLD would attempt this during the chaos of a firefight; are we seriously expected to believe that everybody involved would stand perfectly still, not firing, while someone carefully aimed at a pistol at the weapon in OBL's unmoving hand then shot at it hoping to disarm him? REALLY?!
I would suggest a 'sharpshooter' would be lucky to hit a static beachball at 20m if they were subject to effective enemy fire from the same distance - never mind a dime - simply because nobody would be so bloody stupid as to stand there in those circumstances and sight in on something that relatively small. Absolute idiocy, and completely non-existent outside a Hollywood film.
Check this out:
http://mythbustersresults.com/unarmed-and-unharmed
I actually watched this episode, and of the six times (shown) that they tried to shoot a weapon from someone's hand (replicating the forces involved), the weapon was only dropped twice.
I would suggest luck plays a big part in any firefight, but only an absolute IDIOT with a death wish would choose to rely solely on it for success.