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.
This, combined with the slightly longer barrel, resulted in a slightly improved range but also a lower rate of fire. At the same time, the petrol-fueled auxiliary power unit was exchanged with a diesel-fueled one, allowing it to use the same fuel as the towing vehicle.