First introduced in Resident Evil 2, weapon modification has played some role in a number of titles over the years, ranging in scale and concepts.
In the early titles, the concept of weapon modification was limited to combining Custom Parts with a handgun or shotgun to increase its firepower or handling. Resident Evil Outbreak later introduced a method whereby weapons could be built by combining others (for example, using vinyl tape to fuse an iron pipe and Battery together will make a stun rod). Resident Evil 4 introduced its own method of modification, whereby the player must pay an in-game currency to customise particular elements such as firepower. This was carried on in Resident Evil 5; Resident Evil: Operation Raccoon City and the Chronicles titles. Resident Evil: Revelations brought in yet another format of customization; this time, a set number of custom parts could be used on a specific weapon, leaving the player to decide on which aspect he would have to drop in favour of another.