Some good tips on carrying a spare mag.
My thought is that the percentages just don't require it.
Chance that I'll ever have draw my weapon = 1 in 1000.
Chance that I'll ever have to fire my weapon = 1 in 10 instances of having to draw
Chance that I'll have to expend more than 17rds = 1 in 100 instances of having to fire OR
Chance that my carry piece will have a malfunction = 1 in 1000 instances of having to fire (1 failure in a 1000 rounds, a pretty conservative estimate, really considering that a not all failure require a mag change, just tap rack bang)
So we're looking at about a literally 1 in a million chance of having to fire more than 17rds, and 1 in 10 million of having a malfunction.
If that kind of probability requires that I carry another mag, I should also start carrying a lightning rod. Chances of being struck by lightning are estimated something along the lines of 1:700,000.
You forgot this...