Got me to thinking. What would it take to set up to be able to make modern ammunition after a "crash"? It would be a great survival occupation for a group of folks, both for personal use and as barter. Can smokeless powder be made with low tech means? Manufacturing brass cases? Gilding for jacketed bullets?
All this was done with 1900's technology. Finding and stockpiling the resources can start early.
With the proper knowledge, machinery, and raw materials, anyone can manufacture just about anything in a pinch. The problem is the amount of equipment that is necessary, along with the quality of the final outcome. It would be extremely difficult to produce brass cases of the same quality of the major manufacturers. Modern smokeless gunpowder is made in a very elaborate manufacturing process to maintain lot to lot consistency. Some of it is actually produced under water for safety reasons.
I think a much better alternative would be to heavily stockpile components. For example enough .223 brass that would fill a couple of 30 gallon trash cans would keep a safe full of AR-15's running for a long time. Especially when you consider the fact they can be reloaded several times over. Smokeless gun powder is safe to store in very large quantities, as are primers. Bullets can be cast, just as long as you have access to a large enough supply of lead and Tin. Jacketed projectiles are heavy, but take up very little space.
When you consider how much room you would need to manufacture this stuff, on even a small scale, it would be far easier to use that room to stockpile most everything you would need to reload for years to come. Also, if the need arises it is far easier to move raw components than it is the machinery required to make them. Bill T.