The Allen & Thurber M1837 Pepperbox cannot manually target or even be fired at any target, no error is given. Pawn will not even attempt to target enemies well within range.
Core, Royalty, Ideology, Biotech, and dependencies only in the load order.

Many of the weapons seem to be incapable of manually targeting or automatically aiming at enemies, I can't find a specific pattern based on the list of weapons included in the module.
For example, all three Cooper Shotgun type weapons can manually target and fire on enemies, as can all three Volcanic weapons, whereas none of the Colt weapons seem capable of manually targeting or firing on enemies.
Gatling guns are also incapable