All of the differences here are more visible for the third person muzzle flashes, all the muzzleflashes are generally smaller, but some individual weapons have slight differences in their muzzleflash effect compared to what we have in the game now.
MP5 and Brenner use the same muzzleflash effect, but MP5 also produces slightly blue lighting. (this also applies to the shield's pistol)
the M308 and Bronco have this little smoke puff ring when firing (admittedly this one is a bit harder to notice depending on what heist you're on)
AMCAR/AK just have smaller muzzleflash effects, unsure what's different with B9-S, Crosskill, and MAC-11 muzzleflashes, but i included them anyway.