Love the idea! Off the top of my head:

"The term "assault rifle" is an innacurate and pejorative term. I'll talk to you when you actually learn the subject matter of this debate."

"the second amendment is a guarantee against tyranny, remove the guarantee, invite tyranny."

"Anyone who says that an ar-15 have no value as a hunting rifle has never used one to hunt."

"Mass shootings don't happen with CCWs present because the criminal doesn't make it far once opposed."

"Cities with the most firearm restrictions tend to have the most firearm crime."

If asked "what do you say to the families who lost loved ones in Newtown/aurora/Virginia tech/ columbine?" "An armed law abiding citizen could have reduced or prevented your loss."

"There are already highly restrictive laws on ownership and use of firearms. Perhaps we should consider enforcing those before we try to enact more legislation that will have little or no effect."

"Since the federal firearm ban expired, gun violence actually declined dramatically in the United States."

"Limitation of magazine capacities will functionally have no effect on the ability of an shooter who is I un-opposed by an armed guardian."

"Mass murders more deadly than any shooting can be accomplished with diesel fuel and fertilizer. Disarming citizens will not stop these crimes, it will just change how they happen."

An 'extreme' but personal favorite of my own (this may cross the callous line): "Too many Good Americans willingly gave their lives securing our freedoms to diminish them because you fear for your own."

I am more than happy for you to edit/criticize these. We need to have the best material possible.