(Via Say Uncle)
So, the balloon has went up. It makes no difference what balloon it is, it's up. You, the prepared prepping survivalist, have your food, your water, your medical gear and your guns. You have everything you need to make a stand of it against the Mutant Cannibal Zombie Unwashed Bikers.
And then your AR/AK/HK/Main Battle Rifle clone of choice breaks something, like a spring. Or we are a long way down the rebuilding road, and something just wears out. You aren't going to hope in the truck and go to Bubba's Gun Shop and get spares, and nobody has seen the Big Brown Truck of Happiness with a box from Midway in months or years.
If you didn't stock some spare parts for the things that are most likely to break or wear out, you may find yourself up that infamous creek sans paddle, and it will suck to be you. Even having a spare gun, you still want spare parts.
Shooting Illustrated has a list of the spares you need for an AR.
Interestingly enough, I looked to see if there were similar articles for other rifles, and outside of asked/answered questions on various firearms fora (with their inherent questionability), I found a grand total of zip. If any of you know of a good, reliable list, stick it in the comments, please. I'll make sure it gets pulled up here.