Up to .36g, your range will actually be better the heavier bbs you get, since they will retain a higher velocity over range.
Use .28s if your gun is 400-420. 420-460 use .3g, 460+ use .36s as a general rule... Once you get over 500 you can consider .43g
Make sure they are high quality precision milled like Maruzen or Digicon
Jim