For educational purposes only.....

[RetiredRR]

"A shot of whiskey" In the old west a .45 cartridge for a six-gun cost 12 cents.
So did a glass of whiskey. If a cowhand was low on cash he would often give
the bartender a cartridge in exchange for a drink. This became known as a
"Shot of whiskey".

[DockShootinJack]

That's and interesting piece of wild west trivia