The new Heavy Rifle is available to purchase from Ammu-Nation. Got an Armory in the back of your Agency? Then you can pick up the Heavy Rifle, Compact EMP Launcher, and Stun Gun at a discounted rate.
The Armory also provides access to Mk II Modifications, a Gun locker for storage and customizing your loadout, a Heist Equipment and Armor stand and free Health and Ammo.