One reason the M203 was replaced by the M320 was the loading method.
The M320 swings to the side to open, as seen here:-
The M203 slides the launcher forward to load, which limits the length of any grenades you can load into it.
The variation in grenade length can be seen here:-
The shortest “standard” round would be easy to load into a M203, the longer pair would be manageable (about the size of some smoke/flare rounds) and the longest – looks like a fragmentation grenade with longer payload section – wou