It is legal to make a firearm for personal use here in the US. It is illegal to manufacture one for resale.

Guns that are bought from and individual (not a dealer under a FFL) as used are also not traced or licensed. Also in the US you only need to be an adult to buy ammunition. They even sell it at hardware, general and big box stores. All components to make your own ammunition is also readily available. There are a wide range of gun powders, primers etc... that are available to anyone of age.

I guess it may be an issue in other parts of the world but it's a complete non issue here in the US.