when a sawn off Bk-12 or Bk-18 had a chambered round before being sawn off, the ammo count after will show 0, looking like the round has been emptied after the weapon's barrel got cut off.
when this happened to me earlier i tried to load them both thinking i didnt load them initially, but it doesnt load a round when holding R nor do the weapons accept a round when combining them.
I had to double tap R to unload the round and then reload it so it wouldnt show a 0 in the hud tab where my weapon info is