The best way to do that is to go into matchmaking, go to cooperative, score attack, and pick sniper mode. this mode gives all your weapons unlimited ammo if you not good with sniper no problem pick up something easy like the needler it unloads fast can hit your target even if they move and reloads fast. here is my history of the game i played getting it
http://www.bungie.net/Stats/Reach/GameStats.aspx?gameid=8855...