I just completed a little project I had laying around for some time.... Upgraded my sig 552 with metal bushing, guarder sp100 spring and a tanio coba rifled barrel AK47 lenght.
Average FPS with .25g is about 320 but the range and accuracy is better than my 600fps upgraded APS2. Plus the mid power spring related to an upgraded ROF and lighter load on my 9.6V 1700mha battery.
total cost of upgrade is about 120$ and the results are more than satisfying
Vondnik, team Bad Karma, PQAC
Murphy's second rule: Nothing is impossible for the man who does not have to do it himself.