i need to know what to change a bullet on a Tommy gun

Could you be more specific? What exactly do you want to change about the bullet?

you could call another kind of ammo I guess... or make your own using
AddAmmo = Round
then the specifics of your bullet.

Find the tommy gun's entry.
Find it's magazine line.
Find the magazine's entry.
Replace the bullet in the magazine with what you want.

If your talking about color theres a contract and add green, red, and blue which I use often in the index of the weapon.