I did a quick search on the internet and found a legal USA web site that sold not only many models of AR-15, but also 50 round drum magazines to suit them. This is obviously more than the 30 round magazines we used in the Australian Army in the 1980's.
In full automatic mode that 30 round magazine lasted probably less than 5 seconds. That's probably a good thing if someone is firing back at you at the same time. Those rounds probably also cost AUD$1 each, so it's an expensive activity. The AR-15 is sold in semi-automatic mode only, but it is easily converted to full automatic (I guess it's similar to the F1A1 SLR that could be converted to full auto with a matchstick).
On the other hand, if you're shooting a deer then the right thing to do is hit it in the heart and lungs and kill it with the first shot. Anything else is just cruel.
So that leads me to ask why any civilian needs a magazine that takes more than, say, three rounds. If you're on a rifle range you don't even want that - a single-shot bolt action will give more accurate results. A fifty round magazine only has one use...
But let's deal with some numbers.
Terrorism in the USA
On 11 September 2001 a number of aircraft were flown by al-Qaeda hijackers into buildings in New York. About 3,000 people were killed in this atrocity.
Shootings in the USA
According to the Sydney Morning Herald,
"Each year nearly 100,000 people are wounded or killed by gunfire in America"Motor vehicle accidents in the USA
According to USA Today
In 2005 the number of people who died in motor vehicle accidents in the USA was 43,510. As of 2009 that was down to 34,000.The motor vehicle improvement is impressive. Perhaps some further re-prioritisation is in order.