[MaciekA]

I would be very careful in characterizing the appropriate pairings in durometer-to-fps relationships, because it is inherently flawed in the context of high-RoF or fast trigger response AEG builds.

It is possible to build an AEG with an M160 or M190 spring and to yield (on purpose) velocities well below 400fps. This is important because when trying to cure PE, people are often resorting to these kinds of methods, and curing PE goes hand-in-hand with other things you need to fix when you're building a high-RoF setup, like fixing AoE. Hence the warning.

The commonly-shared guideline should be revised to express spring strength vs. durometer. If I were to build a testing rig for this, I'd be doing two tests:

1) measuring "net piston head offset" vs. spring strength vs. durometer at rest.
2) measuring rebound distance and time during high-RoF fire for various spring strength vs. durometer combinations.

All that said...

For most actual game-legal setups that are not pushing far past 400fps/30rps, there isn't much to worry about either way when choosing between 40d and 70d. I have run 40d on M130 setups and prefer it for the dampened acoustics.

If you can't tell the difference in acoustics, 70d is always a safe bet.