Firearm modification is commonly done in order to enhance various aspects of the performance of a firearm. Reasons for these modifications can range from cosmetic to functional, and can be simple operations that the owner can perform, or complex operations requiring the services of a gunsmith.
Common modifications include:
Some commonly customized firearms include the Ruger 10/22 carbine, the M1911 pistol, and the AR-15 rifle, all of which have large numbers of aftermarket parts suppliers. Shooting disciplines such as bullseye pistol and benchrest shooting also rely extensively on custom-built and modified firearms to achieve top accuracy.
Care must be taken when modifying firearms, as some operations may convert a legal firearm into an illegal one. An example of this is the restrictions in the US National Firearms Act, which define restricted classes of firearms. Examples of illegal firearms under this act are:
These restrictions vary from location to location; some areas may restrict magazine capacity or require special safety devices such as loaded chamber indicators, integral locking mechanisms, magazine disconnects. Also commonly required are identifying marks such as serial numbers or firearm microstamping components, removal of which may be illegal[4] [5] [6] Furthermore - conversion of semi-automatic weapons into select-fire weapons has been illegal for non-SOT holding gunsmiths since the passage of the Hughes Amendment in 1986. Such weapons may be held only by law enforcement and military only except for "dealer samples" left in the hands of SOT holders. Fully automatic weapons converted to semi-automatic only operation are machine guns per the NFA. The BATFE's policy is "Once a machine gun, always a machine gun."