When are crits calculated?

Crits are calculated on hit, using the player's crit-rating at the time of impact. Crit-rating is not attached to the shot itself when it is launched.

However, the Sharpshooter buff lasts for one second after a crit (it says this in the tooltip) so if multiple shots hit within that one-second period after the first crit, they will all benefit from the buffed crit-rate. Missile-dampening makes it much more likely that this will happen.

The dampening sphere itself is just a visual cue, so missiles 'hitting' the sphere only causes them to slow down, nothing more.

As already said, Sharpshooter doesn't drop until something is hit, this may also be a door or other object. The Missile Dampening field isn't a object or monster, so it doesn't reset the Sharpshooter buff.

But when you shoot several Nether Tentacles or Ball Lightnings into the Missle Dampening, they are going to be slowed, so that they actually hit a single enemy somewhere around 4-8 times with one projectile. As the Sharpshooter Buff remains active 1 sec after critical Hit, it can occur that in this one second an enemy is getting hit by many hits caused by Missile Dampening and this makes them go down very fast even if you don't crit for sure on subsequent hits. After this initial second the damage output from those slow arrows in combination with the missle dampening gives an immense damage output on all enemies in the way.

PS. if they have the reflect damage prefix, you're gonna have a bad time, cause you'll kill yourself if you don't pay attention and go invisible at the right time.

Remark: Nethertentacle won't hit multiple times since there is patch 1.0.3 out. Ball Lightening still has the same effect

It's worth noting that Sharpshooter doesn't drop until you actually hit something. (ie: if you stand in town firing into air, you stay at 100% crit).

Due to missile dampening, more hits would have hit the mob in a shorter space of time (due to the dampening meaning you end up with 3-5 shots inside the dampening field) in addition to these shots hitting multiple times each due to slower travel speed*.

^{* This has been subsequently fixed in a patch, and abilities like Nether Tentacles will now only hit each enemy once}