I have no idea how you would reload cartridges above, say .50 BMG. You are not going to run larger cases through a hand press
Blackpowder can be substituted for smokeless, but it is not a 1 to 1 ratio and there are some other things like wadding and filler that need to be utilized to duplicate pressure curves. I wonder how hard it would be to make cordite in a cottage industry? In all reality, manufacturing some form of smokeless powder is certainly not impossible.
Primers will be more difficult to manufacture/obtain, but their are some "field expedient" methods and suitable substitutes. I think the Anarchist's Cookbook might delve into some of them.
The basic projectile will be easiest. Casting lead bullets is very easy...jacketing them with old brass casings (you can jacket cast lead .22 bullets with spent .22 LR cases to make jacketed projectiles for .223/5.56Nato reloads.) Making 20mm and larger projectiles that will travel well when fired is another story...If you have the right raw material, you could mill it on a lathe, I suppose. Much more detailed process, but not impossible.