A gun with a 2 inch-long barrel is going to be pretty useless at not much more than a dozen feet. Good for mugging somebody I guess, but not self-defense or fighting the next revolution. And you can mug somebody with a knife.
Use metal for the start of the bullet where the stress is highest, then switch to plastic for the rest of it.
you could easily design a 4 inch version with rifle grooves on the inside of the casing. it's just another 2 inches!