Cayagad,
I have seen cad and chrome plating come off steel but never nickel except on a few very old pistols exteriors. It does seem odd. If you know someone w/ a borescope that would be neat.I have used a few Olympus ones for photographing inside rocket turbo pumps and they are neat! Mabe a local college! I'm sure if the trigger assy and stock was off they might inspect it for you.
SHills