Also, I just realized this:
"Light pistols can take top- and barrel-mounted accessories", "Heavy pistols can take top- and barrel- mounted accessories", "Machine pistols can take top- and barrel-mounted accessories", "SMGs can take top- and barrel-mounted accessories", "Assault rifles can take top-, barrel-, and underbarrel-mounted accessories", "Sniper rifles can take top, barrel-, and underbarrel-mounted accessories", "Shotguns can take top, barrel, and underbarrel accessories", "Machine guns can take top, barrel, and underbarrel accessories", "Both assault cannons and launchers can take top and underbarrel accessories"...
None of those weapon categories actually say they can take mount-less accessories..