I guess it's time to go through my retelling of "how ammunition gets manufactured" again.
Source: Me. I know the plant manager at Lake City (who was brought out of retirement), and have friends who currently work for both Olin in Illinois and Remington in Arkansas.
Let's say that you're a big, full-line ammo manufacturer. We'll ignore the precursors to making ammo (drawing, forming, and annealing brass, extruding and capping shotshells, manufacturing bullets, powder, primer, shot, wad, and buffer, etc) and only focus on the ammunition assembly process.
Functionally, you have 3 different kinds of ammunition lines: centerfire, rimfire, and shotshell.
A centerfire line can functionally make any centerfire round. From .25 acp to .50 bmg, changing the caliber of what you manufacture is just a matter of changing the tooling (the dies, if you will). Realistically, there are dedicated lines for straight wall pistol, necked pistol, and then several lines for straight wall and necked rifle, with some machines being optimized for large cartridge sizes, and some designed for small cartridge sizes.
This is important, because you can convert a .45 acp line to make .40, or .357 sig, or any other similarly sized pistol round by changing out tooling, which takes a few hours, and is typically done overnight by the 3rd shift maintenance crew. Likewise, rifle cartridge lines are (relatively) easily swapped around.
So, what does this mean in practice?
The manufacturer forecasts demand, figures out how much of what they need to make based on that forecast, and then churns out the rounds. 10 million .45's, followed by 30 million 9mm's, followed by 500 thousand .380's, so on an so forth.
Shotgun lines, on the other hand, are dedicated to the bore (or gauge, if you will) for that round. 12 bore lines only make 12 gauge ammo, 20 lines only make 20, etc. Different manufacturers have different policies for making the more esoteric sizes (10, 16, 28, .410), but the only effective changes are in wad, powder type, powder weight, and shot size/weight. They make what they need, and then make whatever's next on the order list (1 oz loads of #9 for skeet, followed by 1 1/8 loads of #7.5 for field, etc).
Rimfire, however, is a different beast.
First of all, it's dangerous. Very dangerous. Since you don't have a nice (relatively) stable primer, you have to get your priming compound into the brass. This means you're dealing with a liquid that, by design, will explode if it gets smashed with sufficient force. And you're in a room full of equipment who's only purpose is smashing things. So, the rimfire rounds are made off in their own little world, by machines that are totally different in composition and maintenance from those that make centerfire and shotshell. AND, to do maintenance on some parts of the line, you have to shut the whole thing down, air it out, and let it sit for a defined period of time where it does nothing.
Moreover, it's dedicated. It can only make what it can make. If you have a ton of demand for 9mm, you can just dedicate more centerfire lines to making 9mm. The same isn't true for your rimfires.
So, your capacity is your capacity, and can't be expanded without investing literal tons of money (in that if you got it in $1 bills and weighed it, it would weigh WAY more than 2,000 pounds) in space and machinery, and that's all before you get into the people element.
And then there's the time element. You have to build buildings, order tooling, install and test it, and train new people on how to use it. Plus, you have to beef up your supply lines to feed this new operation.
How long, you ask? About a year, give or take, if you're really pushing it. Cost? Upwards of $30 million.
