A barrel adjustment and retaining assembly includes a barrel body having an end surface, a seat body having a protrusion, a limiting component and an adjustment barrel. The seat body is movable relative to the barrel body. The protrusion is moved toward the end surface when the seat body moves from a pre-assembly position to an assembly position. The limiting component is clamped between the protrusion and the end surface to deform such that an outer diameter of the limiting component is increased when the seat body moves from the pre-assembly position to the assembly position. The adjustment barrel is sleeved slidably on the barrel body. The clamped limiting component is confined within a hole portion of the adjustment barrel.