What percentage is it?

[JerryM]

I think that if you have a failure at the ranges statistics say are average for a gunfight you will not have time to reload or clear a jam.
Yes, I understand that one can conjure up a scenario where he would need one and have time to change mags. But one can also conjure up a scenario where he will need two or three.

I do not think that there are statistice, but in one's own experience how many times with the first magazine have you had a malfunction that needed an extra magazine?

If I had such with my guns I would not carry that particular gun.

If one wants to carry a spare have at it. That is different from a need. I think the need is zero for a CHL holder and an attack where he would have to use deadly force.

Jerry

[DefensiveCarry.com]

[TOF]

One grain of sand in the right place can stop a round from being loaded. If you are cocked and locked you are good for one round. It may slow the BG down but it often times takes more than one to stop the aggression.

Your backup mag just might be what stops it.

[Ivan4x4]

I only carry an extra mag for my colt defender when i'm Not carrying my BUG, I figure if i had a failure it would be faster to get my BUG out of my pocket holster then to dump the bad mag get the new mag and load it and chamber a round.

[Sticks]

If it can happen, it will happen...statistically at the worst possible moment.

I carry a spare because I have always carried a spare, counterbalance and extra ammo.

Due to my profession, I am more likely to have a spare magazine failure due to grit getting inside the mag and locking up the follower. I am faced with the conundrum of repeated unloading, disassembly and reloading of the magazine causing premature stress on the spring.

[ElMonoDelMar]

I carry a spare because I have always carried a spare, counterbalance and extra ammo.

This is another good point. I find that carrying a spare is actually easier than not carrying one. It does balance out the belt quite a bit.

[farronwolf]

I have always just based that on what I have read by Nonte & Hallock and various other assorted professional gunsmiths.
They state that bad or defective mags are the #1 cause of semi-auto failure.

I'm not sure where or how you would find any published statistics.

#2 would be an improperly fit, tuned, and/or tensioned extractor.

Thank you QKShooter for addressing the actual question asked. Now, I can actually go and look at the information provided to learn more.

I figured when I posted this, most of the responses would be based on opinion, but it is nice to know someone has information to look into further.

[Jason Rogers]

I think maybe mags are not the number one failure "on the street". It's just that it's the one thing we can only address by replacement in an actual shooting. We make sure our gun is clean and lubed. We go to the range to make sure we don't limp-wrist or cause some other operator induced failure. When we clean we inspect our extractors and ejectors for wear or cracks. We have our firearms inspected by gunsmiths periodicaly throughout their lifetimes.

We do all these things but untill we pull the trigger with our favorite mag in the weapon we can not know if the last time at the range we bent the feed rail. So we carry a spare. Is the # 1 thing hyperbole? Probably. But it is certainly a field expedient repair for a common and unforseeable problem.-Jay