There are a range of non-magical modifications that can be made to mundane weapons. The listed costs are for including these modifications on nonmagical weapons.
Source: Tome of Secrets
Rifling is the process of grooving a gun barrel to cause the bullets to spin in a tight spiral. A pistol or musket with a rifled barrel has a range increment double a normal weapon of its type. A weapon with a rifled barrel is always a masterwork weapon, and costs an additional 450 gp (which includes the 300 gp masterwork cost). Rifling must be applied during the item’s creation; it cannot be added later.
Cost: +450 gp (includes masterwork weapon cost).