In order to use standard NATO ammunition, the FH77B was developed. While the sliding-block worked well with the cased charges acting as a seal, it was decided that bagged charges required an interrupted screw breech.
To utilize standard NATO ammunition, the FH77B cannon was developed.  The original design with a sliding block mechanism worked well with cased charges, but it was necessary to adapt the system for bagged charges by incorporating an interrupted screw breech.