We will be using the guns from the Iron Kingdoms.
Reloading in combat will be a concentration check, using the weapons craft check plus your armors armor check penalty to determine the DC.
For example the check penatlty for a militay pistol is 8 and the armor check penalty for chainmail is -5 and for a small steel sheild is -1. You roll a 4 your class level is 4 therefore you total is 8-6=2 you fail by 6
Failure of 5 or more jams the weapon.