Air Arms Owners Club

Would using a weaker valve spring mean you would need less hammer for the same results?

Simplistically, yes, but there are other effects, especially if you are intending to make other modifications. You are changing the relative force on the valve from plenum pressure and spring pre-load, so the power curve will change.

There are tuners who will tell you AA design for the S4/5xxx series is spot on, others will say it can be improved. If you are fitting a reg to an unregged model, and that involves increasing the size of the transfer port, I can see the point in changing springing and hammer weight.

What are you trying to achieve?

My mk2 pro target. Altering the reg and hammer I can get the fps I want but it's noisy.
Reduce reg and hammer, fps drops and it's quieter. Just thought a lighter valve spring with the lower reg would allow a lower hammer tensions well. I have a reg tester and can set it accurately.
It's a mk3 reg

I can't help you on this one - no knowledge of this gun and regulator. As a general observation, I'd say you need to look out for loss of air efficiency as you reduce reg pressure.

You could do away with the valve spring altogether. Then mess about with the hammer spring to get the optimal

BBF