> Without guns, all other methods I know of are painful and non-instant:

Suffocation with inert gas (e.g. N_2) using mask with exhalation valve is safe, painless and leads to very fast loss of consciousness. Distress associated with suffocation is not caused by missing oxygen, but by excess CO_2, so it does not happen if a person can exhale freely. It sometimes happen in industrial accidents, people lose consciousness without noticing anything.