In order to do this cheat you must aim the tag rifle at the sky hold in the R trigger holding in the R trigger (also reported as press X+B) cycle thru your weapons 2 making sure to land back on your tag rile then let go of the R trigger now you have unlimited ammo for it. If you shoot the shots left will go to 0 but you will still have unlimited ammo.

P.S if you change weapons after you did the cheat it will go away and you will have to do it over again.