Why do we say ‘lock, stock and barrel’?

It’s believed the term was used by a US Senator in the early 19th century, who argued that muskets should be manufactured in three separate parts.

By keeping the wheel- or flint- lock (the firing mechanism), the stock (or handle) and the barrel apart, the transportation and repair of guns would be easier.

It always intended, hopefully, to reduce theft of the weapons in transit. Then once at their destination, the three parts could be combined to make the whole, finished thing.