Here is an updated with addition and deletions.
Massif is an adjective in most cases. I think it was taken out of context. I removed it.
Culasse is standing breech.

Acier - steel
Acier damas (also "damas") - damascus steel
Acier trempé - quenched steel
Acier revenu - tempered steel
Amortisseur - pad (as in buttpad)
Anson et Deeley, Anson- box lock
Arme de poing - hand gun
Arme d’épaule - long gun
Armurier - gun shop owner
Armurerie - gun shop, not a maker, historically a body armor vendor
Arquebusier - Gun maker
Artisan - artisan
Avantage (a droite, a gauche) - cast-off, cast-on
Bande - rib
Prolongement de la bande - extension of the rib (to the receiver)
Balle - bullet or slug
Bascule - receiver/action
Bille acier - steel shot
Bois - wood
Bois de noyer - walnut
Noyer blond - blond walnut
Boitier - housing
Bouche - muzzle
Bouchon de percuteur - Firing pin bushing
Bourre - wad
Bretelle - sling
Bretelle a enrouleur - retractable sling (like on a Ideal)
Brisure, a - Breakage
fusil a Broche- pinfire gun
Bronzage - barrel blacking
Busc - comb (for a stock)
Cale - wedge (in a double rifle)
Canon - barrel
Canons frettés - sleeved barrel set
Canons juxtaposés - double barrel; SxS
Canons mixtes (Arme a) - cape gun
Canon rayé - rifled barrel
Canons superposés - over/under
Canon monobloc - Barrels made in one block of steel
Canon lisse - smoothbore
Capuche - metal piece surrounding the Deeley forearm pushbutton
Carabine - rifle, carbine
Cartouches à plombs - shot shell
Cartouches à balle - slug shell
Chambre - chamber
Chien - hammer
Chiens extérieurs - external hammers
Choke - Choke
Demi choke - Modified choke
Clé - key
Clé d’ouverture - top lever key
Clé sur le dessus - top lever key
Coffret - case (to put the gun in)
Corne - horn
Coquille - action ball
Crochet - barrel hook
Crochet du canon - same
Crochet encastré - dovetailed lump
Crosse - stock
Crosse anglaise - English stock
Crosse à joue - stock with cheek piece
Crosse demi-pistolet - round knob grip stock
Crosse pistolet - pistol grip stock
Crossier - stock maker
Culasse - standing breech or bolt
Culasse mobile - movable breech (i.e. on a Darne)
Damasquiné - damascened
Demi-coquelle - half cock (for hammer guns) (???)
Demi-bloc - chopper lump
Détente - trigger
Devant - forearm
Devant a pompe - Deeley pushbutton forearm
Double detente - double tigger
double verrou de fermeture - double locking bolt
Etui - brass (for a cartridge)
Ébène - ebony
éjecteurs automatiques - automatic ejectors
Extracteur - extractors
Épreuves - proof
Express (also double "express") - double rifle
Banc d’épreuves - proof house
Double-epreuves - double proofed
étranglement - choke
(arme a )Feu central- centerfire (gun)
Fermeture en "T" - Jones underlever
Forage - bore (barrel)
Frette - monoblock
Fusil - firearm (normally a shotgun but also a military rifle)
Fusil de chasse - shotgun
Fusil a percussion - muzzle loading, percussion rifle
Fusil a piston - percussion gun
Gachette - sear
Gachette interruptrice - safety sear
Graveur - Engraver
Gravure - engraving
Grenadière - sling swivel
Goupille - round pin
Goupille de charnière - hinge pin
Guidon - front sight
Hausse - back sight
Illisible - illegible
Jaspage - case coloring
Joue - cheekpiece
Juxtaposé - side by side
Levier - Lever
Levier de sécurité (ou sécurité) - safety lever
Longuesse - forestock
Longuesse a poussoir - pushrod (forestock with pushrod release)
Monodétente - single trigger
Moulé(e) - molded
Noix - Hammer axis
Or - gold
Or fin - fine gold
Oreille - Sideclip
Percussion centrale - center percussion, center fire (hammer gun)
Percuteur - Firing pin
Piece d'armurerie - piece of firearms manufacture ???
Piston (Fusil a-) - percussion gun
Planchette - back sight folding leaf
Plaque de couche - butt plate
Platine - side lock
Platine Holland - front lock as made by H&H or others
Platine arrières - back locks
Platine avant - front locks
Platine encastrée - embedded locks
Platine démontable - hand removable sidelock
plomb - lead shot
Poinçon - stamp
Poinçons réglementaires - legal stamps
Poinçons d'épreuve - proof marks
(Crosse) Poncée huile - sanded with oil finish (stock)
Pontet - trigger guard
Pontet à volute - trigger guard with volute in front
Portée - range (as in shooting distance)
Poudre sans fumée - smokeless powder
Poudre Pyroxylée - smokeless powder (French)
Poudre Vive - smokeless Powder ( Belgian)
Poussoir - push rod (forestock)
Quadrillage - Checkering
Quadrilleur - Checkerer
Rallonge de crosse - stock extension
Rayé à chargement par la bouche - muzzle loader rifle
Ressort - spring
Ressort à boudin - Coil spring
Sûreté - safety
Stéphanois - from Saint-Etienne region
Tire-cartouche - extracteur
Tirette - action rod
Tonnerre - place where the ignition happen
Type anglais - English style
Verrou - lock, also bolt
Verrou de fermeture - closing lock
Verrou vertical - rising bite
Verrouillage - locking system
Vis - Screw