- 01: Introduction
- 02: History
- 03: Propellants, Firearms, and Ammunition Development
- 04: Modern Firearms Manufacture
- 05: Small Arms Ammunition
- 06: Evidence Handling Procedures
- 07: Equipment and Instrumentation
- 08: Examination of Firearms
- 09: Cartridge and Shotshell Examination
- 10: Characterization and Evaluation of Fired Projectiles
- 11: Bullet Comparison and Identification
- 12: Gunshot Residue and Distance Determination
- 13: Toolmark Identification
- 14: Communicating Results
- Resources
Final Assembly
Home > Modern Firearms Manufacture > Assembly > 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 revolver’s 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.
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.
Dummy ammunition round
The final assembler does not usually test fire the firearm. However, if the assembler is building a repeating firearm, dummy 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.
