Glitch- 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.