The 21' rule (Tueller drill) is not rigid BTW, when it was created it was shown that it took the average police officer 1.5 seconds to unholster their side arm, take aim and fire, in that 1.5 seconds the average person could close a gap of 21 feet and be standing nose to nose with that officer. However even at further distance a person can still be shot multiple times and not drop dead instantaneously (and therefore still be dangerous). Which is why LE agencies train their personnel to treat anyone with a knife or other sharp/edged implement as a Deadly threat.
And not all 'bangers' use guns, there are plenty of incidents to this day of thugs carrying/using knives for their purposes.
edit to fix spelling and to add this
http://forums.blueline.ca/viewtopic.php?f=10&t=20566
And say, this isn't that far off topic. If you are going to discuss, whether or not people should have the right to carry use/firearms in self defence, you should at least discuss the possibilty that it may not be the most prudent or ideal weapon to use in self defence.