Firing weapons that use weird firing mechanics like the Angel Arm "reload". This allows the use of angel arm in challenges which restrict weapon type.
yes, and that also makes the Trigun and Beretta behave oddly with dualreload / Gun Kata. making their "specials" not "reloads" would kill two birds with one stone.
I feel that the nuclear weapons are less abusive since they self-destruct and thermonuclear bombs are generally allowed anyway, but they could be changed too for consistency.
edit: on second thought, just making all of those special actions use the alt-fire instead of alt-reload (replacing their current alt-fires, thus also making them more different from the normal weapons) might work. it would prevent use in weapon-related challenges, since you can't fire them.