Without guns, all other methods I know of are painful and non-instant: hanging, drug overdose, cutting veins, suffocation, what else?
It’s just sad that terminally ill people have to use a method that will put stigma and shame on how their live ended, doing it in secret and with mess. Instead of a legal and graceful procedure, going away surrounded by family.
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.
At Dignitas they have the person drink a cup of barbituates. There are a few videos of this entire process on YouTube if you're curious enough to watch such a thing. It appears to be generally painless.
> an oral dose of an antiemetic drug, followed approximately half an hour later by a lethal overdose of 15 grams of powdered pentobarbital dissolved in a glass of water
That way you can do organ removal at the same time.