This is a question of kinetic energy, verses the strength of the defender.
a .44mag round (a good stand in for a SR heavy revolver) and a 5.56mm round (typical assault rifle) hit with around 1400 joules of energy.
Or roughly 1000 foot pounds of pressure.
****
I doubt a troll could hold onto an object when hit with that much energy at the speeds of a bullet for reaction time.

Of course, a 2 handed grip on something may prevent him from dropping it.. but that's your call, I would say my self a strength test would hold onto the item - which would also be next to impossible.
But other then that, If you remembered the -4 for the called shot, any lighting, distance, and wound modifiers then you're good.
Burst fire mods would help yes.
Generally speaking, when this type of thing happens in real life (there are you-tube videos of police snipers shooting guns out of suspect's hands) the weapon is usually destroyed by the impact... or at least rendered unfireable.... But SR doesn't really go that deep into damaged equipment that much.... Only really matrix damage and bricking - which can be fixed up fairly quickly.
And Really, how would you feel as a player if the GM shot up your guns every game (breaking them!) just cause he could

Yes, I've used it several times, both as a player and a GM... and it can add some really tense, dramatic scenes when used juuust right