The Rifle is the fourth weapon and last of the light weapons which comes with 35, 2 upgrades can be bought: movement speed and damage increasement.
The Rifle delivers medium damage, has a slow firing rate, and comes with 35 ammo.
The Rifle should be bought once enough cash has been acquired after buying the Shotgun. The only upgrade that should be bought would be damage increasement; once the Plasma Cannon had be purchased.