system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are AK47s and other Kalashnikov weapons so renowned? How do you make your weapons simpler and hardier than the other guy?

How do you make your weapons simpler and hardier than the other guy? Why did these weapons become so popular?
gpt
The simplicity mostly comes in the form of HUGE tolerances (clearances... sorry) between parts inside the receiver.  If you open an AK up, there's a ton of empty space inside the receiver, and very few moving parts in the trigger group.  That allows you to put all kinds of dirt, gunk, snow, sand, pebbles, dust, etc. inside the thing without gumming up the works.

They're also incredibly easy to maintain because of these loose tolerances and the relatively few parts.  If you can open the receiver, dump out any crud that's accumulated in there, and slather some motor oil over the moving parts, you're good to go. 

As to how they became so ubiquitous, that's partly because of the easy manufacturing process (the receiver is stamped from a single sheet of metal and bent into form), and the fact that the USSR absolutely *loved* to stick its nose into other countries' business; even moreso than the USA did.  They had a habit of mass producing AKs and arming little pissant rebel groups all across the globe, and the gun worked quite well for that purpose because it's so easy to maintain, and so resistant to damage and jamming.  A barely-trained nobody could be turned from peasant to warrior with the addition of an AK. 

So basically, it comes down to the fact that the AK was easy to make, easy to maintain, and tough as hell because that's what it was designed to be.  The USSR war ethic at the time was all about mass production of overwhelming force, and the AK was designed to fit in that niche.