The different methods of making parts for firearms used in the 19th century leave distinctive superficial markings which are often preserved on the inside surfaces of lock mechanisms. These are interpreted by comparison with reference surfaces prepared with hand and machine tools. In all the cases studied, handwork can be distinguished from machined surfaces, and the marks left by some, but not all, kinds of milling operations can be recognized. Examination of locks from representative arms shows that, while there was increased use of machine tools at armories in the United States throughout the 19th century, hand filing was used to bring parts to their final dimensions. The quality of handwork increased throughout the century; in working metal, it appears to have been superior in accuracy to that which could be attained in production by machine tools, but in woodworking, there is evidence that machines were able to supplant handwork by mid-century.