If you police your brass and were able to manufacture gunpowder...
Something easier to do on the range but not in combat, unfortunately.
I agree that making "new" ammo would be difficult, but if you already have the gun, wouldn't you try making the ammo for that rather then making new guns?
It's all about sustainability. Turn 1 has your Civ starting out after 20 years after the Doom of Man(tm). So your people are going to be a mixed bag of survivors, most of which will have a wide variety of weapons. Plus, but turn 80, you'd have about 2.5 generations of growth so you might now have more people than old firearms to arm them with.
Plus think of it in terms of scale. If you have to produce machined brass cartridges, you will have to secure the brass, have the equipment to machine the brass as well as make the gunpowder for the rounds. Now you might be able to make gunpowder pretty quickly, but machining cartridges in large numbers might take some time, especially when there are a lot of other pressing needs. Plus you need a lot of quality control to mimic ammo needed for modern weapons.
So why go through the trouble when you can make your own Minié ball ammunition that use lead? First off, depending on your equipment, what ever size Minié ball you make, you adjust your rifle production to match the caliber you choose. Again, making a copy of a ACW US Army Springfield isn't that hard for anyone who knows how to make metal. Since this is 'old' technology, it is easier to find how to make it, especially with the popularity of blackpowder firearms as well as reenactors. (Do they have any of those in Australia?)
Another factor is simply time. As I said earlier, there are people in Peshawar in Pakistan that are making hand-made copies of the Lee-Enfield rifle so you could easily make a Springfield. These same machinist could also make the ammo for it unlike the AK-47's they also make by hand.
Not to mention, if you went back further to flintlocks, your men can make their own lead shot, even in enemy territory since it's just about melting some lead, pouring into the mold and cleaning up the next morning. So as long as your gunpowder supplies hold up, ammo isn't the problem. I wouldn't want to go on a attack into enemy territory with modern weapons that I couldn't reload unless back at home. That's setting yourself to get wiped out but a bunch of guys just using crossbows.