Bit of advice please a freind wanted his rifle chrono so i went round, the pellets he was using were bisley magnum heavy 21.4 grains , few shots through the chrono the calculations were the highest readings were 17.5 fps the lowest 16,25 fps , when doing the calculations with rws superdomes 14.5 grains the highest reading were 11.5 fps and the lowest readings were 11.2 fps, the point im making is that my rifles i have are all under 12 fps and im sure that if i used the bisley magnums the power would be alamingly higher, does he need to tune the gun down and if so, is it an easy job to do?