Guns are not dangerous enough in d20 systems. Check this out.
When shot, save vs. Spells (or target’s choice between reflex or fortitude) or receive a mortal wound. You die in 1d6-2 turns; if the result is 0, you die in 1 round per Constitution. If the result is -1, you die instantly.
For small caliber, the save is at 2 advantage, and if you make your save you take 1d6 damage.
For normal caliber, the save has no modifier, and if you make your save you take 1d8 damage.
For heavy caliber, the save is a 2 disadvantage, and if you make your save you take 1d10.
Burst fire makes the shot +1 to hit, OR (if that is not needed) 2 disadvantage to the save.
Shooting full auto takes a full round, and allows shooting 1d4 targets that are in melee range of each other, or if aimed at one target, makes the shot +2 to hit, OR (if that is not needed) 4 disadvantage to the save.
Reloading is a move equivalent.
Body armor adds +2, +4, or +6 to the save and cuts the damage in half even if the save is failed.
Every size bigger than medium is +2 save, every size smaller is -1.