[MultipleParadox]

Spring swap to reduce velocity works in AEGs only, or spring powered pistol/guns

It's not going to reduce velocity at all with a weaker spring; you're most likely prevent the slide to cycle properly, or it will make it take longer to complete a cycle

short answer: no.

A different valve in your mag would help, but I doubt there's any that would allow you to do exactly that actually. I'll let other pitch in their ideas as I don't have any viable solution in mind.