The invention concerns a pump for compressed gas for a replica weapon for the projection of balls, comprising: A cylinder (7) forming a gas retention compartment (72) and provided with a cylinder head (71) forming a lateral wall of the said retention compartment, said cylinder head comprising a central orifice (73) A gas ejection nozzle (9) having a first end opening into the retention compartment (72) and a second end opening into a barrel (1) of the replica weapon, and A piston (8) provided with a piston head (81) which is movable in the retention compartment and suitable for compressing the gas in the said compartment, in which, on the one hand, the piston head is conical in shape and, on the other, the cylinder head has the shape of a funnel, complementary to the conical shape of the piston head.