This is important.

AK 47s are known for their reliability.  There are videos of guys on Youtube literally filling their guns with all kinds of stuff and still having them function.  Some examples HERE, HERE, HERE, and HERE.  As previously mentioned, the reason for this is loose tolerances.  Everything has enough clearance in it that it takes a whole lot to make it jam.  In fact, even jam won't make it jam. 

There is only one place on the entire gun that actually needs to be precise, and that's the headspace.  In simple terms, headspace is the amount of room the cartridge has to move fore and aft in the chamber.  If your headspace is too tight, the bolt won't lock closed, causing it to blow up when fired.  If the headspace is too loose, the shell case ruptures, causing it to blow up when fired.  So, it's either right, or it blows up.  On the AK pattern rifles, the difference between "right" and "blows up" is six thousandths of an inch(.006").  That's two sheets of notebook paper thick.

Headspace on most rifles is set by how far the barrel is installed into the receiver.  Many barrels are screwed in, but the AK barrels are pressed in.  A rubber mallet won't work well because the barrel depth is so critical.  A press works best, but I don't have one.  So, I made another special tool.  The main piece is a chunk of grade 8 all thread.  Grade 8 is necessary because the standard hardware store stuff isn't strong enough and will snap.  The barrel end uses some copper washers to keep from marring the barrel, with some regular washers and a nut after that.  The receiver end uses the bolt carrier to press against with an 1/4"  steel plate behind it to keep it from getting marred up.  It works pretty well, just tighten the nuts, and the barrel presses in.  It's a slow process of tighten, check, repeat.  Real actual headspace gauges are available, but are expensive.  Instead, I used a bullet as my "Go" gauge, and the same bullet with .006" worth of aluminum tape on the back as my "No-Go" gauge.  When it's set right, the bolt will close and lock on the Go gauge, and won't close on the No-Go gauge.