I don't know how the 870 works, but I do know the M-16 and Kalashnikov fairly well. And yeah, the mossy 500 is harder to take down than an Kalashnikov.
Seriously, how can you possibly put a Kalashnikov back together wrong? I can't think of a way. I broke my locking lever on the gas tube, but that was fixable with a rock or pliers. I mean, I guess you can do it wrong, but jeez, it's gonna require some work and effort.
Practice your take down more. Remember to slide the bolt carrier assembly all the way forward when putting the trigger group in. It's important, in my opinion, when doing dissassembly to really pay attention to the parts and how they work together.
But yeah, no where near the idiot proof AK or the "some assembly required" AR platforms.