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

Gunsmith

A gunsmith is a person who repairs, modifies, designs, or builds firearms. This occupation is different from an armorer. The armorer primarily maintains weapons and limited repairs involving parts replacement and possibly work involving accurization...

.

Modifications performed

Common modifications include:

• Cosmetic and ergonomic modifications
  • Replacement or alteration of grips or stocks
  • Changing of sights, such as the adjustable iron sights
    Iron sights
    Iron sights are a system of shaped alignment markers used as a sighting device to assist in the aiming of a device such as a firearm, crossbow, or telescope, and exclude the use of optics as in telescopic sights or reflector sights...
    
     or a telescopic sight
    Telescopic sight
    A telescopic sight, commonly called a scope, is a sighting device that is based on an optical refracting telescope. They are equipped with some form of graphic image pattern mounted in an optically appropriate position in their optical system to give an accurate aiming point...
    
    
  • Adding or removing weight to ease carrying, alter balance, or help moderate recoil
    Recoil
    Recoil is the backward momentum of a gun when it is discharged. In technical terms, the recoil caused by the gun exactly balances the forward momentum of the projectile and exhaust gasses, according to Newton's third law...
    
    
  • Metal engraving or custom woodwork
  • Checkering or stippling wood or metal
  • Altering the metal finish, using techniques such as blueing
    Bluing (steel)
    Bluing is a passivation process in which steel is partially protected against rust, and is named after the blue-black appearance of the resulting protective finish. True gun bluing is an electrochemical conversion coating resulting from an oxidizing chemical reaction with iron on the surface...
    
    , electroplating
    Electroplating
    Electroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal...
    
    , or parkerizing
    Parkerizing
    Parkerizing is a method of protecting a steel surface from corrosion and increasing its resistance to wear through the application of an electrochemical phosphate conversion coating...
    
    
  • Use of a shorter barrel to reduce size and weight, or a longer barrel to increase velocity
• Caliber conversions
  • Rechambering or rebarreling for a new caliber, such as conversions made to chamber a wildcat cartridge
    Wildcat cartridge
    A wildcat cartridge, or wildcat, is a custom cartridge for which ammunition and firearms are not mass produced. These cartridges are often created in order to optimize a certain performance characteristic of an existing commercial cartridge.Developing and using wildcat cartridges does not...
    
    
  • Use of a caliber conversion sleeve
    Caliber conversion sleeve
    A caliber conversion sleeve is a device which can be used to non-permanently alter a firearm to allow it to fire a different cartridge than the one it was originally designed to...
    
     or other readily removable insert to fire a smaller, less expensive cartrdige
• Operating mechanism conversions
  • Muzzleloading
    Muzzleloader
    A muzzleloader is any firearm into which the projectile and usually the propellant charge is loaded from the muzzle of the gun . This is distinct from the more popular modern designs of breech-loading firearms...
    
     or paper cartridge
    Paper cartridge
    Paper cartridge refers to one of various types of small arms ammunition used before the advent of the metallic cartridge. These cartridges consisted of a paper cylinder or cone containing the bullet, gunpowder, and, in some cases, a primer or a lubricating and anti-fouling agent...
    
     to metallic cartridge conversions, such as the Allin conversion
    Springfield Model 1866
    The Springfield Model 1866 was the second iteration of the Allin-designed trapdoor breech-loading mechanism. Originally developed as a means of converting rifled muskets to breechloaders, the Allin modification ultimately became the basis for the definitive Model 1873, the first breech-loading...
    
     or Lund conversion
    Kammerlader
    The Kammerlader, or "chamber loader", was the first Norwegian breech loading rifle, and among the very first breech loaders adopted for use by an armed force anywhere in the world. A single shot black powder rifle, the kammerlader was operated with a crank mounted on the side of the receiver. This...
    
    
  • Conversion from fully automatic or select fire to semiautomatic
    Semi-automatic firearm
    A semi-automatic, or self-loading firearm is a weapon which performs all steps necessary to prepare the weapon to fire again after firing—assuming cartridges remain in the weapon's feed device or magazine...
    
     only or manual operation, or vice versa
  • Short recoil to blowback conversions, used in various .22 Long Rifle
    .22 Long Rifle
    The .22 Long Rifle rimfire cartridge is a long established variety of ammunition, and in terms of units sold is still by far the most common in the world today. The cartridge is often referred to simply as .22 LR and various rifles, pistols, revolvers, and even some smoothbore shotguns have...
    
     conversions, such as the Colt Ace
    Colt Ace
    The Colt Ace or Colt Service Model Ace was a firearm designed to allow inexpensive and low-recoil practice while maintaining the feel of the military Model 1911 pistol....
    
     and J. A. Ceiner conversion kits.
  • Conversion from handgun
    Handgun
    A handgun is a firearm designed to be held and operated by one hand. This characteristic differentiates handguns as a general class of firearms from long guns such as rifles and shotguns ....
    
     to carbine
    Carbine
    A carbine , from French carabine, is a longarm similar to but shorter than a rifle or musket. Many carbines are shortened versions of full rifles, firing the same ammunition at a lower velocity due to a shorter barrel length....
    
     or rifle
    Rifle
    A rifle is a firearm designed to be fired from the shoulder, with a barrel that has a helical groove or pattern of grooves cut into the barrel walls. The raised areas of the rifling are called "lands," which make contact with the projectile , imparting spin around an axis corresponding to the...
    
    
  • Addition of a rifled barrel to a shotgun
    Shotgun
    A shotgun is a firearm that is usually designed to be fired from the shoulder, which uses the energy of a fixed shell to fire a number of small spherical pellets called shot, or a solid projectile called a slug...
    
     to allow it to better fire shotgun slug
    Shotgun slug
    A shotgun slug is a heavy lead projectile, that may have pre-cut rifling, intended for use in a shotgun and often used for hunting large game. The first effective shotgun slug was introduced by Wilhelm Brenneke in 1898, and his design remains in use today...
    
    s
• Accurizing
  Accurizing
  Accurizing is the process of improving the accuracy of a firearm or airgun.For firearms, accuracy is defined as the ability to hit exactly what one is aiming at...
  
  
• Sporterizing

Some commonly customized firearms include the Ruger 10/22

Ruger 10/22

The Ruger 10/22 is a semi-automatic rimfire rifle chambered in .22 Long Rifle. It has a removable 10-round rotary magazine which allows the magazine to fit flush with the bottom of the stock. Higher capacity magazines are also available...

 carbine, the M1911 pistol, and the AR-15

AR-15

The AR-15 is a lightweight, 5.56 mm, air-cooled, gas-operated, magazine-fed semi-automatic rifle, with a rotating-lock bolt, actuated by direct impingement gas operation. It is manufactured with the extensive use of aluminum alloys and synthetic materials....

 rifle, all of which have large numbers of aftermarket parts suppliers. Shooting disciplines such as bullseye pistol

Bullseye (shooting competition)

Bullseye, also known as conventional pistol, is a shooting sport in which participants shoot handguns at paper targets at fixed distances and time limits. A number of organizations, including the NRA and Civilian Marksmanship Program in the United States, have established rules and keep records for...

 and benchrest shooting

Benchrest shooting

Benchrest shooting is a sport in which very accurate and precise rifles are shot at paper targets from a rest or bench from a sitting position. Benchrest shooters are notoriously detail-oriented and constantly trying to further the accuracy potential of the rifle through experimentation. Nearly...

 also rely extensively on custom-built and modified firearms to achieve top accuracy.

Legal issues

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

National Firearms Act

The National Firearms Act , 73rd Congress, Sess. 2, ch. 757, , enacted on June 26, 1934, currently codified as amended as , is an Act of Congress that, in general, imposes a statutory excise tax on the manufacture and transfer of certain firearms and mandates the registration of those firearms. The...

, which define restricted classes of firearms. Examples of illegal firearms under this act are:

• Rifles with overall lengths under 26 inches, or barrels under 16 inches in length
• Shotguns with overall lengths under 26 inches, or barrels under 18 inches in length
• Machine gun
  Machine gun
  A machine gun is a fully automatic mounted or portable firearm, usually designed to fire rounds in quick succession from an ammunition belt or large-capacity magazine, typically at a rate of several hundred rounds per minute....
  
  s
• A conversion to a fixed cartridge of over .50 caliber

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

Firearm microstamping

Firearm microstamping, ballistic imprinting and ballistic engraving are all names given to a controversial technology that has been developed with the goal of aiding in ballistics identification; it involves the use of laser technology to engrave a microscopic marking onto the tip of the firing pin...

components, removal of which may be illegal