TargetTalk

All the weight of the pistol is held by the middle finger. Specifically, the bone of the second joint.
After 30-40 shots, the area starts to become quite sore. (even with long breaks after each 10 shots) I've put some spongy double tape on the middle finger rest on the pistol, but it seems it;s not enough.
How do you alleviate this problem? What do you put in the pistol so the finger bone doesn;t rest directly on the wood?

The weight should be across the second section of bone, not the joint. Your grip may be too small. That section of the finger should be centered & perpendicular to the bore. Some grips may also have finger grooves that interfere with nobby knuckles, and may require trimming.

Ditto Doug. The weight should be supported by a fairly large area of finger. See what's marked on the pic.
There are plenty of articles all over the web about grip design / modification. You might want to start with Nygord's Notes.

i see... the thing is my pistol is a FWB100. And the middle finger rest has a huge gap in the middle for the adjustments screws...

original: current: how i grip:
i saw this picture online ... but i cannot tell what he put in there (cork?) and if there are other practices

To my untrained eye and experience with my own grip, it looks like his grip is too narrow for his middle finger. Try adding to the area to move this pressure point to the desired position.

If that were mine, I would remove material from the underside of the upper right (to relieve pressure on the first knuckle), and add material to the underside of the upper left (to transfer weight to the second knuckle). This would give a much better platform to support the gun. I would not worry about the open space below the trigger.

Definitely take out whatever that shim is you have on the right. The goal should be to feel a bit of weight evenly on that finger. If you grip the gun, then relax, does the gun tilt forward and the barrel moves straight down? Or does it deflect to the left? I predict the latter.