What weight pellets are you shooting 270 shots at 760 fps with?.... Is the tank only down to 1500 psi at the end of the 270?.... The reason I ask is that If you are using 7.9 gr. pellets, that is 10 FPE, times 270 shots, for a total of 2700 FPE from one fill.... You are using a 13 CI tank, and assuming a pressure drop of 1500 psi (3000 down to 1500) that is 1500 / 14.5 = 103 bar.... That means you are using 103 x 13 = 1345 cu.in. of air.... 2700 FPE from 1345 CI of air is OVER 2.00 FPE/CI, which while not impossible, is pretty much airgunning nirvana.... I have never heard of that efficiency being achieved in .177 before....

Bob