Standardized parts that (supposedly) required little or no hand fitting and very good machine manufacturing was in place well before the Civil War. Even Hall rifles and carbines were made that way. It was only a question of scale brought on by wartime requirements. In fact, if you examine a Civil War era weapon that is still in good shape, you would be impressed by the level of workmanship evident, all done with machines. The equipment used to manufacture rifles at the Harper's Ferry government factory in Harper's Ferry, Virginia (which it was at the time), was from Pratt & Whitney Machine Tool Company.
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
Buy War Bonds.