Firearms are not used at all in the movies, so we figured there must be good reason for this. We came up with two: Imperial body armour is more effective against kinetic weapons, and firearms do not do as much damage as blasters (which we think are particle beam weapons). This matches notes placed in various Star Wars RPG books, and it makes sense that a galactic conquering empire would have the best weapons available.

For these rules, Earth has two kinds of firearms: pistols and rifles. Pistols include any weapon which fire pistol ammunition (such as the MP-5, UZI, and Thompson SMG, etc). Rifles fire rifle ammunition (includes machine guns such as the SAW and M-60).

Pistols
Earth Pistols do 3D Damage
+1 if > 9mm
+1 if pistol is a magnum
+1D if the pistol is fully automatic (such as a submachine gun)
+1 if hollow point or hydroshock ammunition is used
+2 if black talon or golden saber ammunition is used against unarmoured targets, -2 if this ammo is used against armoured targets

Rifles
Earth Rifles do 4D Damage
+1 if >7mm
+1 if rifle is a magnum
+1 if rifle fires a 2 or 3 round burst
+1D if rifle is fully automatic
+1 if hollow point ammunition is used

Other Related Rules
All Galactic standard hard body armour is doubly effective against kinetic weapons (fist, rock, knife, fragmentation, bullets, etc). This means Imperial Scout Armour adds +1D+2, Stormtrooper and Bounty Hunter Armour adds +2D. A Blast vest is soft armour and so is not doubled.

Earth produces armour piercing ammunition. This rifle (not pistol) ammo cancels the Imperial armour doubling bonus but adds no other benefit.