Once you've found the NCR Mojave outpost, enter it and agree to do a job for Ranger Jackson, Save, then fast travel to Goodspings load up on 9mm ammo. Head to the job site and shoot a bunch of giant ants, it is easy to kill them with the 9mm pistol. Return afterwards to Ranger Jackson inside the NCR Mojave outpost building. He'll give you the service rifle and some other stuff. Save and continue.