Instead of the normal GURPS aimming system, where you add the Acc of the gun in the first second of aiming, then +1 per extra second up to a total of Acc+3, we use this:

Any aiming whatsoever, or an effective skill right as you're about to roll over the weapon's SnapShot number, negates the SnapShot penalty.

Add +3 to your to-hit roll per action of aiming, up until the gun's Acc.  After that, add +1 per action up until a total of the gun's Acc+3.