Well, theres a pellat like this:
Which are plastic.
Then theres:
Which are metal. (and a lot smaller)
If you have a gas gun, just get plastic, or if you want to hunt with them, get metal.
If you just want to have fun, get plastic. Dont hurt and wont cause injury if not shot at the face.
JUST REMEMBER:
BB GUNS ARE DANGEROUS!
AIRSOFT IS SAFE AND FUN!