- I think an m110 should do it. Its not an exact science because there are variations between different manufacturers of springs.
- You could also cut a few coils from the one you have but you also have to heat the end red hot to flatten the last coil and then file it smooth.
- If you get a gun that is only slightly over 350 like 360 or so, you can fire on full auto and stop before it completes the firing cycle. This will keep the spring compressed and leaving it like that for a week or so will reduce the fps. Just test after a week and repeat if necessary.
While not as easy as with a quick change spring system, it should be something you can do yourself. Look for disassembly video on YT
Take your time and maybe take pictures as you disassemble for reference to put it all back. (The videos should be enough tho)