I think the handgonne now shoots grenades is because no one would use a handgonne to shoot bullets when the matchlock is available.
The Koreans initially received a matchlock from Japan as a gift, didn't understand what it was for, stored it away, then started mass producing them once the Japanese invaded and showed the Koreans how effective it was in battle. So I don't want the Koreas to use the handgonne to shoot bullets when they should be using matchlocks.