Normally, you would have to wait until the "Fifth Reputation Upgrade" to snag the Carbine Assault Rifle, but you can get it earlier. First, enter a building, such as a bank. Start shooting, and continue the disruption until finally the police arrive. Find shelter and take out the cops as they come, without getting killed. The SWAT team will eventually arrive. Kill members of the team and pick up a Carbine Assault Rifle they drop. Now, let police officers kill you. When you spawn again, check your weapons locker and find that it now contains the desired gun.