1. In a breech mechanism of the type for use with an automatic or semi-automatic gun comprising a breechblock mounted in a breechcase having a barrel cooperating therewith, said breechblock adapted to be locked by cooperation between the barrel and locking lugs formed on the breechblock, means at the forward end of the breechblock for receiving the rear end of cartridges to be fired, a casing non-rotatably mounted in the breechcase about the breechblock for longitudinal sliding movement, and means for locking the breechblock by transforming into rotary movement of the breechblock at least a part of the longitudinal movement of the casing, relative to the breechblock, which relative movement takes place when the breechblock has substantially reached its front closing position at which it closes off the rear end of the barrel, the improvement wherein: the said means at the forward end of the breechblock comprises a cup-shaped recess, and said locking lugs are formed on and spaced apart about the periphery of the exterior of the breechblock, a first longitudinal recess formed on the exterior surface of the breechblock in a first space between the locking lugs, for receiving a cartridge extracting means carried by the breechblock, and a second longitudinal recess formed in the exterior surface of the breechblock in a second space between the lugs which is opposed from the first longitudinal recess, the depth of the second longitudinal recess into the breechblock being sufficient so that the second longitudinal recess intersects the said cup-shaped recess at the forward end of the breechblock to provide a passage for a cartridge ejector carried by the breechcase, said extracting means comprising an extractor member held in place to engage a cartridge by a heel part which pivotably engages a front part of the breechblock, and including a leafspring mounted in said first recess, the rearward end of this leaf-spring being held against the breechblock and the forward end of the leaf-spring being biased outwardly against said heel part, thus maintaining the extractor member in place both radially and longitudinally relative to the breechblock.

4. In a breech mechanism of the type for use with an automatic or semi-automatic gun comprising a breechblock mounted in a breechcase having a barrel cooperating therewith, said breechblock adapted to be locked by cooperation between the barrel and locking lugs formed on the breechblock, means at the forward end of the breechblock for receiving the rear end of cartridges to be fired, a casing non-rotatably mounted in the breechcase around the breechblock for longitudinal sliding movement in the breechcase and between a forward position and a rearward position, relative to the breechblock, means for forming a generally annular and generally fluid tight chamber between the outside of the breechblock and the inside of the casing in either of said forward or rearward positions of the casing relative to the breechblock, and means for locking the breechblock by transforming into rotary movement of the breechblock at least a part of the longitudinal movement of the casing, relative to the breechblock, which relative movement takes place when the breechblock has substantially reacheD its front closing portion at which it closes off the rear end of the barrel, the said means at the forward end of the breechblock comprises a cup-shaped recess, said locking lugs being formed on and spaced apart about the periphery of the exterior of the breechblock, a first longitudinal recess formed on the exterior surface of the breechblock in a first space between the locking lugs, a cartridge extracting member carried by the breechblock in said recess, and a spring located in a groove formed onto the exterior surface of the breechblock and having one end fixed to the breechblock within the fluid tight chamber and biased radially outwardly of the breechblock axis for urging the cartridge extracting member to its cartridge holding position, said recess guiding said extracting member to prevent lateral movement thereof relative to the breechblock, and a second longitudinal recess formed in the exterior surface of the breechblock in a second space between the lugs which is opposed from the first longitudinal recess, the depth of the second longitudinal recess into the breechblock being sufficient so that the second longitudinal recess intersects the said cup-shaped recess at the forward end of the breechblock to provide a passage for a cartridge ejector carried by the breechcase.