The "silvery part" in the chanel of the action is the main spring on an R model Darne.
I have neither a Darne or Charlin. What I have is a Halifax (License Darne). It has this silvery part in the channel, but on this gun it definitely is not the Main Spring, but simply a bbl latch. The forward end has to be depressed before the bbls can be removed as Ted described, by Bumping.
The Mainsprings on this gun though,which I was under the impression is essentially a Darne R, are housed within the breech block. I have never made an attempt to disasemble the block, but highly suspect the main springs are coil type surronding the strikers.
