Here is a calculator for working pressure.... You need to know the tensile or yield strength of the material.... using tensile will give the burst point, using yield will give the point at which it deforms and doesn't return to the original size.... You have to input the safety factor.... 3:1 on burst or 4:1 on yield is what I use on my own projects.... This only gives the data for the tubing, you have to work out the end plugs separately, and that can get fairly complex, depending on whether they are threaded in or secured with screws.... I didn't notice any bending on the main tube of my Millenium Pumper, even at 2000 psi.... Bob
That FWB 600 has an energy of 5.3-5.8 FPE, which is a LONG way from 12, like less than half.... Once you talk about more than one cocking stroke.... or retaining air in the valve, so that you can have several cocking strokes to fill the gun and then 1 or 2 to top it up between shots, you have a competely different situation.... Springers use adiabatic compression and the heat generated to increase energy, again a totally different concept.... My response was directed at the original concept of an SSP of 12 FPE.... I think it would take a Gorilla to cock it.... Bob