The projectiles (20) are passed along inside a holding tube (18) and fed one at a time into the firing barrel (3); a movable mouthpiece (10) is caused by spring-action means (14) to move away from the initial end (7A) of the firing barrel (3) and, when said mouthpiece (10) is pressed against the mouth, its blow pipe (12) is brought toward the firing barrel (3) and pushes the first (20X) of the projectiles (20) toward it.