Step 1: Construct time machine.
Note: Constructing such a time machine will only add $10 to $12 trillion to the national deficit.
Step 2: Hire deadly assassin who can blend in perfectly in the 19th century.
Step 3: Using the time machine, send deadly assassin back to 1880.
Step 4: Have deadly assassin kill Richard Gatling and Hiram Maxim, thereby "un-inventing" automatic and semi-automatic firearms
*Voila!* Gun problem solved!
Don't laugh. Knowing Harry Reid and Nancy Pelosi, the brain trust of the Democrat Party, this plan is bound to get wide support from the Left.