rearward stroke thereof exceeds a predetermined amount the locking by the locking block 3c is released so that the rearward movement of the entire assembly of the breechblock assembly 3 is effected on the other hand the action bar 7 is restricted its rearward movement by the engagement of the stepped shoulder portion 8a thereof with the flange portion 1a of the gunbarrel 1 so that it is stopped at the position shown in fig6 when the breechblock assembly 3 moves forwardly the action bar 7 is restored to its initial position in the sequence of operations as described above when the value of the gas pressure introduced into the cylinder 4 is large the breechblock assembly 3 might collide against the rearward bottom portion of the receiver housing 2 so that it be damaged and therefore the enlarged diameter portion of the action bar 7 is made large in length while the reduced diameter portion thereof is made short in length thereby permitting the rearwardly urging force given to the breechblock assembly 3 to be reduced by the constructon described above the construction is reduced to be far simpler than the prior art gas operating device of the kind described above and the action bar 7 can be easily interchanged when the gunbarrel 1 is removed from the receiver housing 2 and proper rotational operation of the breechblock can be insured by using an action bar which is appropriately adjusted in its axial displacement for various shot shell having different gas pressure characteristics hereby broadening the range of application of the.