I have read somewhere that the barrett xm109's recoil  which uses 25x59 ammo was beyond human limitation (and maybe because of this was scaled down to 25x40) according to this a regular m203's kick should be even bigger but you see on combat videos (not movie) that it's pretty managable. what's the theory behind the power of these round? is reduced powder comes in the picture?

IIRC you are talking about the "payload rifle".  Small Arms Review mag did an article on it a couple years ago.  i don't remember them saying that it had horrible recoil.  again, IIRC, it isn't a high pressure cartridge so the recoil would be more like a magnum shotgun.  the projectiles don't rely on velocity for affect, but rather what is carried inside.  i haven't heard lately how the project was coming along.  i believe that the marines were working on something similar that could be fired out of the standard issue 12ga.    this is just stuff that i have read over the last couple years and don't have firsthand knowledge.

advntrjnky

here is the Small Arms Review article that I mentioned:

Barrett 25mm "Payload Rifle"

advntrjnky

Has Barret released any payload rifles to the public?