I remember when there was a rumor going around of a guy who took an old shovel and made it into an AK type firearm. Found a link with photos: http://acidcow.com/pics/40255-ak-47-made-out-of-a-shovel-49-...
The harder part in the case of an AK is the parts kit that has all of the other parts that must be actually machined. Step 0 of making an AK from a shovel is "buy a parts kit made from an entire old, previously functional gun minus the sheet metal and wooden furniture". In terms of making a true firearm from scratch an AK would still be fairly difficult. In terms of making a technical "firearm" (the controlled part), the AK is one of the simplest.