presence of the tear band 137 since until that is removed the device cannot fire however there is a potential problem in assembling the device in that the penultimate component to be assembled is the upper sleeve portion 102a which carries the tear band 137 the last component to be assembled is the cartridge 103 and until the sleeve portion 102a is in place accidental firing is possible accidental firing during the assembly process is a real possibility firstly immediately prior to installation of the upper sleeve portion 102a there is a stage in which the partially assembled device has a period of quarantine to check for gas leaks secondly during installation of the upper sleeve portion 102a the device will be subjected to numerous forces and vibration arising from the assembly equipment even after installation of the upper sleeve portion 102a the assembly stresses arising as the cartridge is installed may be sufficient to cause accidental firing despite the presence of the tear band 137 to deal with this problem the device has a safety mechanism in the illustrated embodiment this is provided by forming the slot in the ram not only with the cam surface 109 but also with a locking surface 109a.