Just got a RWS 350 Mag .177 At 1250 fps.You have to use the heaviest pellets available or the accuracy goes all to hell re: blown skirts.Crossman Premier Ultra Mags are made for guns over 1000fps at 10.5gr pellet weight.What they do is slow down the velocity to 1000-1100fps so the pellet doesn't have to ride out the shockwave and gets below the sound barrier to eliminate the sub-sonic crack like a .22 RF [Faster isn't always better]........Harold