"I'd just like to add that you'd be surprised how long some of the oddball calibers of the 19th century held on."
Yep. Even though no guns were being produced, there were still guns in circulation, and the ammo companies would get a couple order a year, and minimal demand was just enough to keep production going.
There were two great ammo line cleansings in the United States in the 20th century...
The first was in the run up to American entry into World War I. First the allies started coming to the United States for ammo, and then the United States needed ammo.
Rounds that went out of production then were the lesser liked and loved rimfire rounds like the .25 Short, the .30 Short, and centerfire rounds like the .44 Evans Short.
The more popular of the obsolete rounds in some cases actually stayed in production during the war because it wasn't a total war effort.
The second great cleansing can be thought of as one big one, or two back-to-back smaller ones -- the Great Depression and World War II.
The Great Depression drove ammo sales to levels not before seen in the United States. Quite a few makers barely made it through the depression, others, like Peters, were gobbled up Du Pont/Remington.
That's when most of the rest of the early rounds went out of production -- .401 WSL, .41 Short Rimfire, .41 Long Colt, .45 S&W, .44 Colt, .25 Stevens Rimfire, the various flavors of .22 automatic from Winchester and Remington...
Dozens more rounds became obsolete.