166 in the event of malfunction an o ring groove type gas seal 180 may be placed between the breech cap 166 and the wall of the breech to provide further sealing as shown in fig3 a valve slider 182 with a first end adjacent the forward end of the housing and a second end adjacent the rearward end of the housing is slidable within the valve passage 122 from a first position adjacent the forward end of the housing to a second position adjacent the rearward end of the housing the valve slider may be partially hollow adjacent its first end and adapted for receiving a valve spring 196 the valve slider may be formed having a first enlarged portion 189 adjacent the second end of the of the valve slider 182 and a second enlarged portion 191 forward of the first enlarged portion 189 as shown in detail in fig4 in a preferred embodiment the valve slider 182 forms or includes seals 186 188 190 with the valve passage 122 at a plurality of points for example in the figures three points are shown for illustration where single o ring groove type seals 186 188 190 provide sealing but multiple o rings or any other appropriate method of sealing.