It works like this first off reloading is a repair skill, so like any skill you have to put enough point into it in order to open up some the specific ammo types. First thing you need to do is decide on what ammo you want to use. I created a character that will use 3 round types .357, .44 and .308, i sold off all the other types of guns hanging on to all the ammo though. As someone said the primers are the [censored], pistol powder is pistol powder as is rifle powder rifle powder, but the primers are small pistol, large pistol, small rifle, large rifle, and the .50 caliber. You get the primers from other ammo you break down which is why i feel it is important that as a player i decide which weapons i want to use early, and then i buy, collect as much ammo of the other types as possible to use for the materials to make the ammo types i will use. My character at this moment has a repair of 55 so i stopped there for the moment as i can make all the rounds i intend to use for the time being.
In order to make .357 ammo i must break down the 9mm ammo i have into the base components, I can break down 9mm into 4 pistol powders and 1 small pistol primer, 10mm breaks down to pistol 5 or 6 pistol powders and a large pistol primer. Since i am using .357 weapons i need to make the ammo and that is 1 small pistol primer and i think 4 pistol powder and some lead that you get either by a recipie that breaks down scrap metal into lead or by recycling the lead from the broken down 9mm or 10mm rounds. Now believe this or not but i even broke down the existing .357, .44 magnum, and .308 ammo i had and remade it into ammo of the same caliber, and as of last night i had fired close to 40 to 50 rounds of my reloaded .357 ammo and have registered very little damage on the weapon, i am not sure but i think the stuff the player makes is better than the stuff you buy any of it. I could be wrong but so far it seems this way. I hope this little bit helps if you still are unclear just hit me again and i will do my best to answer.
On the Wikia site they will give you the base ids for the casings, primers, so i would recommend that you save the game console command in some items and play, that is what i did to figure it out. I then just reloaded the save and played on.
Asai