hole 115 formed at a predetermined position thereon when one of the powder packs 172 is set off to explode in the main body 11 the high pressure gas so produced may be guided into the muffler chamber 112 via the through hole 6 and then be slowly discharged from the through hole 115 with the hollow muffler chamber 112 the sound wave of sonic boom produced in the main body 11 shall be enclosed and reflect within the substantially closed muffler chamber 112 without leaking out there the noise brought along with the explosion of powder pack 172 is thereby largely reduced the thrusting mechanism 2 of the present invention consists of a coupling 21 a sliding block 22 a centre rod 23 a connection member 24 a spring 25 a hollow push member 26 an inner hexagon screw 27 a second connection piece 28 and a sleeve 29 please refer to fig1 and 3 the coupling 21 is screwed to a rear end of the first connection piece 19 of the shooting mechanism 1 and has a middle partition 211 therein all the centre rod 23 sliding block 22 and connection member 24 are received in the coupling 21 the centre rod 23 is a slender rod having a front head 231 with larger outer diameter and a threaded rear end to engage with the connection member 24 the front head 231 of the centre rod 23 is retained to one front.