IMHO, ammo crafting is only worth doing if you have the hand loader perk and can craft the special ammo from there, because standard ammo is common and cheap enough that you don't really get an huge advantage from crafting it.
You can get most of the raw materials from the gun runners though don't expect to get huge amounts of extra ammo that way. But if anything, ammo crafting, handloading perk or not, is good for converting ammo you don't use to ammo you use most often.
Unfortunately you can only break down standard ammo, not the hollow point or armor piercing siblings, nor can you craft HP and AP ammo types, making the the skill less useful overall, especially without the hand loader perk which allows you to craft more specialty ammo types that are actually better than the usual HP and AP ammo.
If you are using the Ranger Sequoia the Brush Gun which uses the rare .45-70 Gov't and/or any rifle that fires .308 rounds, then the Hand Loader perk and ammo crafting becomes much more valuable for the following rounds:
- .45-70 Gov SWC Hand Load
- 308 JSP Hand Load
- The 10mm FJP Hand Load is also handy for the 10mm pistol. If you have 50 sneak it's a great holdout weapon, especially using all the mods and the hand load ammo