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.
The FH77B cannon was designed to accommodate standard NATO ammunition. Although the sliding block mechanism worked well with cased charges, which served as a seal, it was decided that bagged charges required a different design, specifically an interrupted screw breech.