I realize I'm necro-bumpin' the h*** out of this dusty old thread, but figured I would give my input here.
I've been messing around extensively with converting a RAP47 and RP4 METS to pellet guns. After countless hours of experimentation, I've come to realize you just can't get a lot of power from this design. The most I've managed is around 8FPE with 14.3g .22 pellets (16" rifled barrel), and only 6FPE with .177 10g lead balls.
I like to refer to the design as a "cork gun" because of the way the launching rod works. It basically gets pressed over the O-rings on the "steer wind pipe" by the main spring (24lbs of pressure). When you pull the trigger pressure builds up behind the launching rod. When the pressure reaches a certain point, the press/friction fit of the rod on the O-rings finally gives and it goes flying forward, using pressure and momentum to overcome the extremely strong main spring.
The problem to getting more power is that even with a higher PSI air source, the "cork" still pops at the same low pressure. I've tried up to 1500PSI and gained hardly any power (power adjuster internals removed). Even if a higher PSI netted higher power, the trigger becomes impossible to pull since it directly opens a valve at full air source pressure.
I've ordered some 90-duro O-rings to see if I can get the "cork" to seat a little tighter and break free at a higher PSI. If they're too stiff or too big the main spring won't be able to re-seat the launching rod though.
To be continued....