1996 produced FWB 300S, disassembled, cleaned, relubed, new piston buffer/breach seal and a Maccari single spring installed. FPS per a new a FX Chrony set at 0%. Haven't done a fps verification vs another chronograph..
Grain fps
H&N Finale Match Light 7.87 617
AA Diabolo Express 7.87 639
Crosman 7.8 (1,250 box) 7.90 603
JSB Exact Diabolo 8.44 601
JSB Exact Heavy 10.34 501
The only real difference between the 300 and 300S is that the 300S has 2 springs that are installed with the coils wound opposite one another to reduce twist. The single Maccari spring is harsher than the dual springs with a small gain in fps. Pellet on pellet accuracy at 10 yards with the H&Ns. AAs up to at least 45 yards, maybe more. These are 10 meter competition spring powered mechanisms that don't lend themselves to power tuning.
Disassembly, reassembly, cleaning, seal installation and lubing is not difficult. Keep an eye on the C clips, as they tend to launch during removal if not careful.. Lube sparingly.