With all the posts about how long a bolt lasts, or barrel, etc., I found this article interesting.

On bolts, they typically get 20,000 rounds out of their bolts before they see issues... and these are full auto, rental M-4s.

Also, interesting note on Melonite, or nitro-carbonizing, or whatever name you want to go with... The melonited barrel on an HK only went 10,000 rounds before keyholing, where he has had chrome lined barrels that didn't keyhole after 80,000 rounds!

Again, this is full auto and I think the barrel on my Sport is just fine, but interesting none the less.

AR endurance findings, at a rental range - The Firearm Blog