We should draw the line at the energy sufficient for there to be somewhere between 4 and 5 victims of a murderous rampage. How can I allow one murderous rampage and disallow another one? Because we cannot stop all murderous rampages. In particularly we cannot stop any member of a family from killing the other members of their family. Unless we all sleep with an on duty officer in our homes at night we can't prevent it. Whether it be with a gun, a knife, a fire, or other act of negligent stupidity, a family will die and it will be tragic. If we say the average family is 4 or 5 you've just done the victim math. The second way you can draw the line at the energy level to kill 4 or 5 in a murderous rage is to examine the most lethal energy we all possess. And that is the automobile. It is within our power to drive our car into a crowd on any given day. Sadly we've been seeing this method of terrorism increase. But what's clear is that this is not that effective a method of obtaining mass causalities. A car if driven into a crowd, even at high speed, will lose energy very quickly. The first few contacts will be fatal. But beyond that the energy will decrease. There will still be a number of injuries but beyond 4 or 5 few will be lethal.
So now that I've established the selection criteria I will do the math and show you the result. Here is a table the compares the lethal energy of a number of fire arms common in the US today. Drawing the line at 4 or 5 casualties it's simple to pull off where I believe we set the standard. Many of you will disagree. The question then is over to you.
If you are having trouble understanding where the line is in practical terms, that is if you buy into my argument regarding maximum lethal energy, it should be clear that assault rifles are out, unless you restrict them to single shot rifles. And high capacity clips or magazines are also out. You can still have hunting rifles, handguns, revolvers, and shotguns. Do we really need AR-15's and AK-47's? And it appears shotguns with large magazines as well as the highest power hunting rifle should also have magazine restriction. It's important to note, our latest US shooter did everything right. He accomplished his goal whatever twisted and demented objective he was after. All the fire power and concealed carry in the world couldn't get to him at his vantage point before he unleashed his murderous rampage. In my opinion restricting his lethal power would have changed the outcome significantly. Pleas note the last row of the spreadsheet shows the lethal energy in the single shot musket loaded with black power and minie ball. It doesn't have the lethal energy to kill in a single shot. This is an important fact should anyone ever want to argue 2nd Amendment rights based on energy alone.
Look at the energy numbers in the table. Please note the caliber of the AR-15 and the 22 rifle are the same. The energy in the AR-15 is 10 times that of the .22. Speed kills just like when driving a car. It's not the individual behind the gun (or wheel) that causes the fatalities. It's the energy that does the killing.
For those of you into math here are the actually energy calculations for a single round fired from each weapon. Weight of each round and muzzle velocity taking from the comprehensive details of Wikipedia.