This war-movie standard phrase has always annoyed me. It even showed up in "Reluctant Lieutenant," a book I’ve been reading, purporting to have been used by sergeants on Basic Training firing ranges at Fort Dix, NJ in 1967. Bothers me, I say, because it’s not obvious to me how one could lock first and then load. But loading first and then locking the rifle’s bolt forward makes sense.
Indeed, the original order was to load and lock and it comes from the M-1 Garand Manual here, the standard rifle of the second world war. But Wikipedia says lock and load also makes sense in terms of locking the bolt back before loading the round into the chamber. In any case, they attribute the current usage to John Wayne in the movie "Sands of Iwo Jima" in 1949. So I suppose it could have been used that way at Dix eighteen years later, and ever since.
THE TEXAS SCRIBBLER 
I also own a business named Lock & Load. The term goes back to the 1700’s. On the Flintlocks, you had to pull the Lock back to load gunpowder in the flashpan. Good to chat with you again.
Sounds reasonable. I didn’t know it was that old. Thanks.
Just a bit of information, since I’m currently researching this expression and found your post: On a flintlock, the pan would open when struck by the flint at firing, so it was not necessary to open the lock. I have a Tower pistol that works this way, and you can see this is consistent with the 1764 Manual of Arms at http://www.outwatersmilitia.com/Manual.html.
The two mostly likely origins of the phrase seem to be the action of taking striker portion of the lock to half-cock (locked position) before loading, or loading the pan in the lock with the first bit of powder from the paper cartridge, whereupon the lock would be closed and the weapon would be turned 180 degrees so the lock faced away from the shooter before dumping the cartridge into the barrel. The former explanation seems less likely since in contemporary usage, the word “lock” appears to have been used more as a noun rather than a verb.
Just a bit of information, since I’m currently researching this expression and found your post: On a flintlock, the pan would open when struck by the flint at firing, so it was not necessary to open the lock. I have a Tower pistol that works this way, and you can see this is consistent with the 1764 Manual of Arms at http://www.outwatersmilitia.com/Manual.html.
The two mostly likely origins of the phrase seem to be the action of taking striker portion of the lock to half-cock (locked position) before loading, or loading the pan in the lock with the first bit of powder from the paper cartridge, whereupon the lock would be closed and the weapon would be turned 180 degrees so the lock faced away from the shooter before dumping the cartridge into the barrel. The former explanation seems less likely since in contemporary usage, the word “lock” appears to have been used more as a noun rather than a verb.
Thanks, DJ. I have little familiarity with flintlocks. But it was really an explanation of the modern usage that I was after, which is detailed, to my satisfaction, here:
https://texasscribbler.com/blog/2008/08/lock_and_load_1.html