A barrel box integral with the barrel rear end, with a hinge and latch for closing with respect to a fork integral with the front end of a pressure chamber. The box being connected to a mechanism of levers for compressing a plunger of the pressure chamber, the fork including an internal fork part obtained by molding, which concentrates most of the complex configurations, tightly fit and joined to a portion of a the mouth of the pressure chamber, which is of metal tube and tightly covers the whole of the internal fork part, the barrel box being also obtained by molding, with a rear part of the barrel as an insert.
