If you register to Rockstar socialclub and link it to your xbox live account, you will have the assault SMG unlocked, available for free in any ammunation and you will be able to upgrade it for free.
EDIT 1 : Thanks o Smeds o for the info.
Make sure you equip everything, tints included. It won't unlock until you have used them all.
EDIT 2 : Thanks ADaughen for the info
Works with Special Edition/Collector's Edition guns, too.
You only need to apply one of the tints; so, just one of each of the modifiers.