Just to give some feedback....I've been inside at least 7 FWB 150/300s...replacing bumpers, rings and breech seals and of course various springs and spacer combinations....my goal has been more speed with JSB 7.33gr pellets (cuz in all FWB guns tested this is the accuracy/speed pellet...heavier JSBs are equal but without the speed).
The Gotha piston ring made no difference in any gun tested....
The Gotha export power spring did increase velocity over the factory double springs...but
The Maccari springs were far superior in speed increases.
All springs tried gave an initial big boost for about 150 shots before velocity slowly backed down and settled in about 25 fps slower.
So a gun bumped up to 695 fps eventually settled down to a 670fps gun...a power spacer picked up an additional 10 fps.
Gotha is a great source of parts...but Maccari's springs work best for me...and his $25 300S spring while giving a good 50fps boost, is not my first choice to get a high speed boost.
Really nice work on the stocks displayed in this whole thread....I let others collect em...I'm modifying and shooting the ones I get..