The lock refers to the firing mechanism. Throughout the history of muskets there have been a number of different variations of the lock mechanism, such as the matchlock or flintlock mechanism. The stock refers to the wooden butt end of the musket. The barrel refers to the barrel, obviously.
In the military muskets shipped to various locations with the lock, stock and barrel separated. I’m sure when the package arrived there was a sticker indicating “some assembly required” but their probably was a good instruction manual included. If you were a civilian and you wanted a gun you would have to obtain all three parts of the musket separately and then have them put together. With the onset of the Industrial Revolution you could go to a gunsmith to buy a fully ready weapon and you didn’t have to go through the process of obtaining the lock, stock and barrel separately.
|Muskets firing in a volley|
When you hear someone say lock, stock and barrel you can be thankful that you no longer need to assemble your own gun but you can buy a preassembled one. Wait, is that what we should be thankful for?