I could see getting an adapter to do something like, 20g to .410 gauge, but 20 gauge to a pistol caliber....
You might as well just use the right tool for the job. An actual handgun.
At least then it will be accurate and you will be getting the proper ballistics out of the round.
TACAV: Griz, if you had to choose between bacon or coffee...
grizcty: You left out the choice of death