Play the Fireman chapter in Eternal Mode. Pick up the rifle, select it as your weapon, and select "Mode" until there is a picture of an ammo-clip next to it. The order is one bullet, three bullets, clip, grenade rounds. Since you never lose any bullets, you can now shred enemies without prejudice by simply holding Attack. When locked-on to zombies, you can blow off all of their parts by wiggling the Analog-stick during your relentless spray of bullets. You can even keep hitting them after they fall to the ground by staying targeted on them.