Making bullets is trivial. It's black powder, a case of metal (brass, aluminum, etc), and some molten lead.

This doesn't even address the constitutional right. You can't ban the printing press and claim it doesn't affect the freedom of speech.

Aside form high powered stuff you can get away with pure lead bullet.

See https://news.ycombinator.com/item?id=47771707 for the rest.

You can re-use the shells, so all you need to do is cast the bullet, which is really easy, then load the gunpowder into the shells and use a simple machine to crimp the bullet on, and you're done. There's lots of off-the-shelf hardware to do it that is pretty common throughout the US.

Many people already load and reload their own bullets in the US because it is significantly cheaper. Good brass can be used many times over and loading equipment consists of an arbor press and some dies. Regulation would make getting it harder and more expensive, but all you would be doing is creating an ammunition black market that funds criminal enterprise with a supply still too large to do much in hampering gun crime.

Responsible gun owners and hunter who practice regularly would be harmed the most because they use tons of ammo. Criminals won't because they might only shoot a few bullets in their life and usually from close range.

They do that too, but they still let people buy motor vehicles, with which they can drive to Nevada or Arizona to purchase ammunition.