Tuning hw50s detune

Depends on how much preload is on the spring, if more than 3-4 coils of preload then cutting a coil off will be fine but if only 1 or 2 coils of preload then a spacer will be required after cutting of a coil. The other option is a spring and guides with a smaller wire diameter. Dropping the power is fine but you must maintain a certain amount of preload to make the firing behavior smooth.
 
I'm going from memory here so...

I recently put a Maccari kit in my 50s. I do remember he had a sub 12fpe kit but I used his regular one. That one gave me a few less fps from factory, but it made the gun smoother shooting and less twang/buzz. The 50 is not that powerful a gun to begin with. I believe my factory spring had 3-4 coils of preload and the new Maccari spring had 2-3, again, from memory.

Anyway, I would recommend the regular Maccari kit and seal. My backyard range is 22 yds, but I'm dead on from 15 to 30 yds with the ftt 11.42 in .20 caliber.
 
So, I did it cut two coils off main spring and clapsed polished the end. Went from 760 Fps to 720 fps shooting 7.9 cphp. Cocking is definitely less effort and the shot cycle just as nice. Not quite as loud, going to stay at that level for some time and see how it goes. Definitely like the results. Thanks all for the information and advice. Still has solid 1 1/2 inch preload as well.