When you pulled the trigger on your flintlock rifle and only the powder in the pan went off without igniting the main charge, it was called “flash in the pan.”
If you sold your entire rifle, and not just the parts, you sold it “lock, stock and barrel.”
And of course “straight as a ram rod” came from the the loading rod used to stuff the powder charge down the barrel.
I thought “flash in the pan” had to do with panning for gold.
You posted;When you pulled the trigger on your flintlock rifle and only the powder in the pan went off without igniting the main charge, it was called “flash in the pan.”
No, I believe it originated during the Gold Rush. A flash of gold in the pan looked promising but in the end the claim didn’t pan out.
Speaking of flintlocks, I think you “went off half cocked” not to mention “hang fires”.