What is the realistic life span of a gas tube? In auto or semi? What are the reasons for failure? Wear at the carrier key resulting in gas leakage? Cracking? Those that have worn them out or had them fail care to share the details?

Gas tubes last a pretty long time. I usually just replace at re-barrel time. I am a highpower shooter so I re-barrel at 3500 to 5000 rounds depending on barrel, steel, rifling, etc. type. Cost is not an issue as they cost around $7 or so. However, I would not replace until I started to get functioning problems as, like you said, where the carrier key rides up on the tube there is a little nib. And if you get gas blow by there, you may get functioning problems. One thing that tends to extend gas tube life is making sure the carrier key rides up on it just right. If it doesn't you get poor accuracy and less life. Also, keep those damn pipe cleaners out of it.

Ditto rebel's comments, they last a long time in "semi-auto" rifles, but they tend to wearout faster in "full-auto" rifles, so if your shooting SA one spare gas tube per rifle will keep your AR/ARs viable until TEOTWAWKI.

Provided ya also have the other major wear part spares stored away.......

Mike