AirMax P15 .25

:4:
Last string looks good, glad to see. Time to finalize break in, find the best pellet for this setup and have lots of fun!

By cutting off 5mm, you definitely decrease preload but with that also the force the hammer hits the valve, therefore theoretically less fps keeping the same regulator setting. It’s the same as screwing out the hst by 5mm.
The benefit of decreasing preload is decreasing the risk of hammer bounce and therefore inconsistency. Some manufacturers use springs without any preload, and even have some room to slide in the uncocked gun. When you shake it, it rattles a little.

In order to have the same force for your hammer with a shorter spring, you’d need to have a spring with thicker wire/different material.
From my experience with the cricket, a stainless spring with given dimensions (length/outer diameter/wire thickness) is weaker than a piano wire spring with the same dimensions. Piano wire spring in turn is likely to give a little “ping” sound.
I have shortened the Cricket spring so there’s no preload at all but increased wire thickness to maintain force.
You might need to readjust your reg as well, as said, it’s a balance.

surprised as the shot string on the end of page 1 after greasing pretty much looked regulated.
Let me try to share some of the basics I learned.
In general, a regulator works with pressure difference. You fire the round, the regulated pressure drops, the piston in the regulator opens and admits air from the high pressure part of the air cylinder. As soon as the pressure behind the regulator (plenum volume between reg and firing valve) rises, the reg piston begins to close and shuts off at a certain set pressure (the regulated pressure).

The closing force (regulated pressure in the plenum volume) works against the washer stack. As soon as you fire a new round, the closing pressure falls, the springs are opening the piston until the inflowing air pressure closes again, and so on, and so on.

Knowing that, it becomes clear that normally screwing in the set screw decreases regulated pressure, as this closes the gap between the piston and the seat (it closes earlier). On the other hand screwing out increases the gap, the regulated pressure increases, as the piston closes later.

Increasing the gap too much results in a regulator that will not close at all, the full air cylinder pressure passes through, therefore you get an unregulated shot string.

Not knowing exactly the assembly of your reg, I keep it that general, hope it helps nevertheless.

Hammer spring works against the regulated pressure. There is a balance between both. Regulated pressure too high vs hammer spring : fps go down, as hammer spring can not open the valve long or far (one is related to the other) enough.
Regulated pressure too low vs hammer spring: the plenum empties completely and fps depends on reg pressure only.

Best would be starting to increase hammer spring force a max and try set your regulator slightly above the desired fps, then decrease hammer spring tension to reach desired fps, as a first approach. Fine tune from there.