Honestly I don't think you could make a good casing out of hemp compound, but for the bullet itself, you could make a jacket.
The itself bullet is usually cast out of lead, and it's covered in what's called a jacket, which makes it more solid for penetration. Generally the jacket of a bullet should be a metal which is somewhat soft, so that the spiral cut pattern in the gunbarrel (called rifling), can dig into the bullet and force it to spin, giving the bullet a gyroscopic spin is what makes it accurate. Nylon, or a theoretical hemp compound, could have the right rigidity to do the same thing as copper.
You can do this without a jacket, just a pure lead bullet, but it will get dirty faster, because lead will wear off into the barrel. Generally making a bullet jacket out of aluminum would be questionable at best.
The casing is what it sounds like, it's what contains the powder load, the primer, and what holds the bullet.
While brass is what's commonly used (because it works the best), aluminum can be a low cost alternative, as can steel. Brass casings can be recycled by the user (clean them up, trim them slightly and load new powder, primer and a bullet in it), steel casings are generally considered not to be worth reusing due to the extra wear on your tools, and aluminum casings can't be reused at all.
Bullets themselves could be dug up from the ground and reused (because it's ridiculously easy to melt down and recast lead), some gunranges will dig up their shooting berms to gather up lead that's been collected in there over long periods of time.