be adjusted to the interior surface of chamber 3 and a final perimeter edge 2 c fig3 shows the barrel 2 in the resting position in which the sealing elements 2 b 2 c of the rear end 2 a of barrel 2 face the chamber 3 which contains a pellet 9 a the thin cylindrical wall 2 b is arranged so that when the barrel 2 is moved backwards by the trigger 5 see fig4 the thin cylindrical wall 2 b enters the chamber 3 collecting along the way said pellet 9 a which is located in the chamber 3 until said final perimeter edge 2 c is supported against the rear surface 24 of the chamber 3 making sealed contact around the compressed gas entrance 25 while said exterior surface isolates the chamber 3 from an opening 8 a in the ammunition magazine 8 typically said ammunition magazine 8 comprises an ammunition store 19 for a column of pellets 9 where this store 19 communicates with said chamber 3 through said opening 8 a an ammunition push mechanism 20 operated by spring 21 is arranged to push said column of pellets 9 towards the chamber 3 in accordance with this invention a trap 22 jointed with respect to a shaft 23 is arranged in said opening 8 a between said ammunition store 19 and the chamber 3 so that it is pushed upwards by the penultimate pellet 9 b in the column.