depends how powerful you are going, look up some charts as to see how powerful you want the spring. I've put an m100 madbull spring in mine, seems to shoot bbs pretty quick but i havent chronoed it yet, If you're looking to skirmish outdoors, then to stay under the general 350 fps limit a lot of sites have, get a m100 or m110 spring, from a make like madbull, guarder or Prometheus.