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There are two levels of assembly: subassembly and final assembly. Subassembly is the completion of components requiring attachment of multiple pieces. Final assembly creates a finished, functional firearm from the combination of these components.


Parts that usually require subassembly include these:

  • Hammers
  • Triggers (including trigger groups in some classes like semi-auto and pump shotguns)
  • Cylinders
  • Bolts
  • Slides

Subassembly is usually required for major components, such as semiautomatic pistol slides and rifle bolts. Several tasks in the functioning of a firearm are performed by these major components, including housing the firing pin and performing the extraction of the cartridge case. These functions are accomplished by small parts and springs that are attached to the major component. The process of fitting smaller parts to major components allows for a better final assembly flow.

video courtesy of Savage Arms

Final Assembly

The amount of work involved in the final assembly process depends on the design of the firearm. Some firearms will fit together and function without individual fitting of the parts. This is called a drop-in fit, which saves time and labor if it can be accomplished. It is an extension of the concept of interchangeable parts as conceived by Eli Whitney.

Drop-in fit is a goal but not a reality for some gun designs. Even with modern equipment, many gun models require a degree of hand fitting to achieve proper function or appearance. Double-action revolver lockwork has always required skilled hands to create a smooth and predictable double-action trigger pull. The fitting of a revolvers cylinder is also a hand-fitting operation. Even with the best manufacturing techniques, fitting a cylinder into a frame seldom qualifies as drop-in. The cylinder gap and the headspace gap must be established by hand: a skilled fitter usually sets the critical timing of cylinder rotation and locking.

Photo of a dummy ammunition round
Dummy ammunition round
Photo Courtesy of Brownells (see reuse policy).

If the firearm is chambered for a bottleneck cartridge, final assembly may include establishing the headspace of the firearm. Headspace is the dimensional relation of the cartridge to the chamber measured parallel to the bore axis. Headspace is critical to safety and reliability. If the headspace is set too tight, the firearm may not work when dirty. If set too loose, the firearm could misfire or the cartridge case could rupture during firing. A hand-chambering reamer is usually sufficient for the final adjustment.

Whether hand fitted or not, most parts are checked for proper fit, using either physical feel or calibrated gauges. Gauges are used to check headspace and barrel/cylinder gap. The double-action function in revolvers is an example of checking by feel. Properly fitted lockwork will produce a smooth and even trigger pull. Hesitation or jerking indicates improper fit.

The final assembler does not usually test fire the firearm. However, if the assembler is building a repeating firearmdummy ammunition is used to cycle the weapon while watching and feeling for proper function. These inert cartridges are stained dark brown or black for identification and contain no primer.

Final Steps

Upon completion of the assembly process, the firearm is tested before being shipped.

Proof testingtest firing, or both, are performed on the assembled firearm.

Proof Testing

Most major gunmakers use proof testing to stress test their products. Special proof ammunition is loaded to pressures 30-40 percent higher than the most powerful commercial ammunition. One or more of these cartridges is fired in each firearm. The maker stamps the firearm to indicate proof testing was conducted.

the letters S A A M I in green
National Institute of Justice (NIJ) (see reuse policy).

In the United States, members of the Sporting Arms and Ammunition Manufacturers Institute Inc. (SAAMI) perform proof testing to comply with voluntary guidelines. In most other countries, firearms proof is the role of a government agency called a proof house. Each country has its own rules of proof; most follow the standards of the European Commission Internationale Permanente (CIP).

Blue and Yellow logo with C.I.P letters
Video courtesy of Savage Arms

Test Firing

Test firing differs from proof firing in that it uses standard commercial ammunition. The test will show if the firearm functions properly and if it meets accuracy specifications. Some manufacturers use mechanical test results for the sake of uniformity and personnel safety. Different models and cartridges have different performance requirements. The number of test firings and the accuracy specifications may vary across models and manufacturers.

Video courtesy of Savage Arms

Serial Numbers

Serial number on a rifle
Serial number on a rifle
Photo courtesy of Forensic Technology WAI Inc. (see reuse policy).

A serial number may be applied at any point during fabrication; it must be present on all manufactured firearms. The number must be placed on the major component, defining the product as a firearm. Low-cost commercial rifles and shotguns, including those made by major American manufacturers, were not required to be numbered until after the enactment of the 1968 Gun Control Act.



Preparation for Delivery

All that remains is to ready the completed firearm for delivery. Any residue left by test firing is removed from the action and the barrel. Metal parts are treated with a rust inhibitor and the gun placed in a padded container for protection during shipment. Instruction manuals, warning tags and stickers, and other literature are packed with commercial arms.

Video courtesy of Savage Arms

Selected Bibliography

The Selected Bibliography is a list of the writings that have been used in the assemblage of the training program and is not a complete record of all the works and sources consulted. It is a compilation of the substance and range of readings and extensive experience of the subject matter experts.

  1. Biasotti, A. 1981. Rifling methods A review and assessment of the individual characteristics produced. AFTE J 13 (3): 34-61.
  2. Bird, J. Feb. 1996. How handguns are made. Handguns .
  3. Brown, C., and W. Bryant. 1995. Consecutively rifled gun barrels present in most crime labs. AFTE J 27 (3): 254-258.
  4. Brundage, D. 1998. The identification of consecutively rifled gun barrels. AFTE J 30 (3): 438.
  5. Coffman, B. 2003. Computer numerical control (CNC) production tooling and repeatable characteristics on 10 Remington model 870 production run breech bolts. AFTE J 35 (1): 49-54.
  6. DeFrance, C., and M. Van Arsdale. 2003. Validation study of electrochemical rifling AFTE J 35 (1): 35-37.
  7. Hatcher, J.S. 1947. Hatchers Notebook . Harrisburg: Military Service Publishing Co.
  8. Jones, F., H. Ryffel, E. Oberg, C McCauley, and R Heald. 2004. Machinerys Handbook . 27th Edition. New York: Industrial Press Inc.
  9. Lopez, L., and S. Grew. 2000. Consecutive machined Ruger bolt faces. AFTE J 32 (1): 19-24.
  10. Lutz, M. August 1970. Consecutive revolver barrels. AFTE Newsletter.
  11. Mathews, J.H. 1962. Firearms Identification, Volume I . Springfield: Charles C. Thomas.
  12. Matty, W., and T. Johnson. 1984. A comparison of manufacturing marks on Smith & Wesson firing pins. AFTE J 16 (3): 51-56.
  13. Matty, W. 1985. A comparison of three individual barrels produced from one button rifled barrel blank. AFTE J 17 (3): 64-69.
  14. Matty, W. 1999. Lorcin L9MM and L380 pistol breechface toolmark patterns. AFTE J 31 (2): 134-137.
  15. Peterson, H.L. May and June 1960. The development of firearms. American Rifleman.
  16. Rosati, C. 2000. Examination of four consecutively manufactured bunter tools. AFTE J 32 (1): 49-50.
  17. Ruger. March 1997. Rugers gun talk rifled barrels. Guns and Ammo .
  18. Savage Arms, Inc. http://www.savagearms.com/
  19. SigSauer. Owners Manual . http://www.sigsauer.com/CustomerService/OwnerManual.aspx
  20. Sturm, Ruger & Co., Inc. Ruger Instruction Manuals http://www.ruger.com/index.html?r=firearms
  21. Weller, J. Jan. 1971. The quiet revolution in U.S. firearms manufacture, Part 1. American Rifleman .
  22. Weller, J. Feb. 1971. The quiet revolution in U.S. firearms manufacture, Part 2. American Rifleman .
  23. Weller, J. Mar. 1971. The The quiet revolution in U.S. firearms manufacture, Part 3. American Rifleman .

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