got a 9 mm Flobert rimfire/16 ga adapter, made long time by the well known barrel maker Lothar Walther in germany but now out of production for many years. L. Walther made a lot of such adapters that are higly sought after now most times by trap hunter to shoot small game in the trap. you can make your daily tour of inspection with the combination gun in case you need a big bullet or a shotgun and when there is something in the trap you just load the cartridge adapter to shoot it with the small bullet.
could be the same caliber but if you have an adapter shooting 32 ACP from a 308 rifle you don't need another gun.

the chamber for the 9 mm Flobert rimfire is off center so the center fire firing pin hits the rim

try the RWS BB cap, it make 167 m/sec( from a long barrel 240 m/sec) and 36 mm into dry pine wood
hits the point at 4 meter also

but now with the little shot cartridge, same like the .22 rimfire shootshell above, 6 meter from the target, aim at the black dot

surprised by the good patern

to have a comparison I shot my old belgian garden gun the same way, RWS 9 mm Flobert shot

and the pattern is bigger than the short 16 ga adapter had

here left the 16 ga adapter, right the garden gun