Well, depending on how complex you want to get it varies. But for basics...
To Hit:
1 - Complete Miss (May hit Allies instead)
2 - Complete Miss (Will not hit Allies)
3 - Near Miss.
4 - Hit (Gets lodged in hit area)
5 - Hit (Goes through, may hit another enemy)
6 - Hit (Goes through, may hit an ally)
To Determine Where it Hits:
1 - Left Leg/Hip/Foot
2 - Right Leg/Hip/Foot
3 - Torso
4 - Left arm/Shoulder/Hand
5 - Right arm/Shoulder/Hand
6 - Head/Neck
To Determine Severity:
1 - Minor Graze (Little Bleeding, low damage)
2 - Major Graze (More Bleeding, low-medium damage)
3 - Non-Lethal Penetration (Medium Bleeding, medium damage)
4 - Painful Penetration (Medium Bleeding, medium-high damage)
5 - Harmful Penetration (Medium-High Bleeding, high damage)
6 - Lethal Penetration (High Bleeding, High-Extreme damage)
Skills would impact mostly on hit chance. For example, having high gunslinging would increase accuracy, but reduce chance to get allies (Modifying it towards a 5 rather than a 6) at high levels. At lower levels, it would modify towards a 6.
For example, someone with high gunslinging rolls a 6. It gets bumped down a point or two, depending on HOW high, to avoid them overshooting, but still getting a good result.
Someone with lower gunslinging, but still relatively decent rolls a 5, and gets bumped up to a 6 rather than staying at 5 like a higher skill level character would.
Weapon damage would impact on the 3rd set of rolls, modifying it depending on how lethal the gun is.
Weapon accuracy would modify the first roll. In an obvious way.
The 2nd set of rolls would also allow you to introduce aimed shots - With a high enough skill level you could aim for, say, the head, and get bumped a point towards the head. Or if already hitting the head, bump up damage.
Could also use a 'Rate of Fire' mechanic for guns. Say a gun has a rate of fire of... 2. That means in one turn, 2 bullets could be fired from that gun. You could also introduce quick shots, allowing them to do a second action, but with minor negatives to both the shot and the following action, and you could use rapid shots, lowering accuracy/damage for more bullets per turn.
Tell me if any of this is too complex, or if it's interesting or what not.