(Accidentally posted this to the wrong Forum this morning).
Hit locations don't really matter aside from fluff effects until you hit Critical Damage. Once you get to critical damage, then you calculate the effect based on the location.
Example: Acolyte Bob has 8 wounds left. He suffers 5 impact damage to the right arm. He's now down to 3 wounds, but it has no further effect because he's not in Critical.
However, if Acolyte Bob has 3 wounds left, and takes that 5 impact damage to his right arm, then he's reduced to 0 wounds, with 2 critical damage. I will go into the Critical Damage chart to see what happens to him when he suffers 2 critical damage to the right arm.
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Now as far as RAW is concerned, Critical Damage stacks regardless of location (ie. If Bob is at 2 critical damage in the arm, and takes 4 more damage to the left leg, he's now at 6 critical damage to the leg), but nobody I know plays this way. They largely keep the critical damage centralized to where it occurred-- ie. Bob is at 2 critical damage to the arm, and takes 3 critical damage to the right leg, then his leg suffers ONLY the result of the 3 on the critical damage chart, not 5. If he takes that 3 damage to the same arm though, THEN it goes up to 5.
As far as how I keep track, it's not hard. Once a character enters Critical damage, I simply designate the critical damage they've taken with something easy like 4H, 3RL (4 Critical Damage to the Head and 3 to the right leg).
But until you get to the Crit chart, there's no difference between wound location whatsoever.
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As for your question about extra damage, GENERALLY no, but you can do additional damage based on degrees of success with certain weapons. Accurate Weapons deal an additional 1d10 damage for every 2 degrees of success, firing a weapon on Semi-Auto adds an extra hit of the weapon for every two degrees of success, and firing on full auto gives an extra hit for every degree of success.
But for a common weapon without any special rules there are no inherent advantages to hitting with a large number of degrees of success.