Oh boy, just some ideas.
Everyone wants to point to the low number of gun homicides in European countries, some quick counters.
The US has over 3 times the population of Germany, and over 5 times the population of the UK.
European countries have a much lower poverty rate than the US, and studies show that increased poverty = increased violent crime. This explains why the UK and Mexico have the same gun laws, the UK has 50ish gun homicides a year and Mexico has over 20,000.
Violent crime of all types has increased since the UK instituted their gun ban, including gun crimes.
Finally, if they talk about "who needs an assault rifle" ask "who needs a sports car?" Studies show that accident rate increases with engine size, and more young people (26 and under) are killed in accidents involving sports cars than any other type of car, 36% of fatalities.
If they ask why do you need to carry more than 10 rounds, ask why do we need to have cars that do 100mph+? Every argument for banning assault weapons can be turned around to ban sports cars (which kill more people annually than assault weapons do.)
http://www.lbl.gov/Science-Articles/...SUV-report.pdf