I usually show them a photo of an M1-Garand. It is pretty, has a nice wooden stock and to someone who has little to no experience with firearms resembles a hunting rifle.
Once I explain that that is the Gun we won WWII with and that it is significantly more powerful than an AR-15 it tends to open their eyes a bit.
But for the most part I have given up arguing with anti-gun folk. Their arguements are based entirely on emotion. If a gun looks scary then it IS scary. Attempting to explain that an AR-15 is no more powerful than any other gun chambered in .223 Remington is a losing gambit because they are not playing the same game we are.
Attempting to explain to someone for whom the government is a comforting pancea that the REAL purpose of the Second Amendment is to secure the other rights of the people through force against a tyranical government, or that the Battle of Lexington and Concord occured as a result of attempts by the British to confiscate the arms and poweder of the colonists and is an example of the same is useless.
Try showing these two pictures to an anti-gunner
Then ask which one should be banned.
The top rifle is chambered in .577 Tyranosaur one of the most powerful shoulder fired rounds in the world.
The bottom is of course the ubiquitous M-4 carbine available in a semi automatic civilian version.
I can about guarantee that the anti gunner will pick the bottom rifle.
Then show them these
The largest round in the second picture is the same as the SMALLEST round in the first picture!
The smallest round in the second picture is of course a .223 Remington (5.56 NATO) the round shot by the M-4 pictured above. (I know the .223 and 5.56 are not exactly the same)