Well Argo, first of all let me tell you that you did a hell of a job here. I live in Belgium in the Dutch speaking part but I speak also fluent French. I will wait your final version that you gona post and if I have some comments I will try to help you.
For the moment, if I was in your place at the translation of BREECH, movable (Darne)- Culasse mobile . And just because you mention Darne this is more correct " Culasse a mouvement linéaire".
Cheers, greetings from Belgium, Marc.