Denny's right.
If the S ever HTF, full auto is probably the last thing you want; semi is how you want to be shooting. You waste a lot of ammo in F/A and in the SHTF situation, you're not going to be able to call Ammoman or AIM Surplus and get an overnight delivery. And unlike any serious military force, you're not going to have a convoy of trucks to resupply you with extra food, water, ammo, etc. Don't get me wrong, full auto has its place in combat, but when you have no idea how long you may have to go without resupply, you better learn to conserve what you have and use it most efficiently. Full auto is not an efficient way to use ammo. Additionally, the AR15/M16 is not meant for sustained full auto fire. If you're not careful about how you use it, you can fry your gas tube quite easily in full auto and render your rifle useless. Ever try to replace a gas tube under fire? Me neither, and I hope I never do. Ive got a few class 3 goodies in my safe, but for SHTF, the selectors will all be set to "semi" (with the exception of the crew served weapons).