A structure is mounted in a gun barrel and includes an adjustment assembly arranged at a front end of the gun barrel, a connection tube arranged in the gun barrel in abutting engagement with the adjustment assembly to be driven by the adjustment assembly for movement, a plurality of thread sections formed on the adjustment assembly and in threading engagement with the gun barrel for fine adjustment of the adjustment assembly, at least one transmission sleeve arranged at one end of the connection tube that is distant from the adjustment assembly, and a press-down member arranged at one side of the transmission sleeve to be abutted on by the transmission sleeve for pressing inwardly of the gun barrel.