s for engagement with a complementary coupling of the exhaust block 6 the exhaust tube coupling may also have a detent socket 7 d formed through the flange 7 f fig4 a illustrates the exhaust block 6 fig4 b illustrates the exhaust block 6 assembled with the exhaust tube 7 the exhaust block 6 may be mounted near the muzzle and may have a bore 6 b formed therethrough for passage of the exhaust tube 7 and the coupling for receiving the exhaust shoe 7 s the block coupling may have a bayonet profile 6 p formed in a front end 6 e thereof for receiving the lugs 7 k the lugs 7 k may be inserted into the bayonet profile 6 p against the return spring 9 and rotated in the bayonet profile such that the return spring may press the lugs against a locking shoulder of the profile the exhaust block 6 may also have a detent socket 6 d formed in a front end 6 e thereof a detent spring 11 s and a detent plunger 11 p may be inserted into the detent socket 6 d just before mounting of the exhaust tube 7 the flange 7 f may compress the detent plunger 11 p against the detent spring 11 s as the lugs 7 k are inserted into the bayonet profile 6 p and the flange.