Coca is grown outside the US for economic reasons, not because coca can't be grown here. Up until the recent advent of legal marijuana, most marijuana in the US was imported too.
The only strengthens my argument. My point is that just because something can't be produced in the US, that doesn't mean that it won't be transported, sold, and used within the US.
That's because the DEA went to other countries and pressured/assisted their governments in going after the production of methaqualone powder. This strategy be unrealistic and destructive if attempted with firearms (plus similar efforts against coca production in South America haven't gone great either).
Use in suicide is not a reason to ban anything. You think a suicidal person really cares about the instrument they use?
Do you have evidence that guns are used more in domestic violence than in other crimes like homicide, theft, etc?