I think it all depends on what kind of firing cycle you want to end up with Evert.
Between 9.5-10.5 ft lbs can be obtained in .177 with a standard piston and synthetic seal but you do need a stiff spring.
While you have yours apart, please can you check whether the transfer port is straight or angled on your 50.
I have a 50 project going at the moment and it took me by surprise to find the transfer port is angled.
All the best
HW55T