Here's the deal with dimples in an AK:
The "front trunnion" has one hole - each side - that is countersunk drilled. The "receiver sheet metal" is drilled to the center hole size. And then smashed around the edges to match the countersink in the trunnion hole below it. So that you then have a countersunk trunnion hole and a matching countersunk receiver hole. Then instead of using a rivet head that is flat at the bottom neck (opposite side of the head from the top). You use one that has a corresponding countersunk or "swelled neck" to match the countersink in your receiver/trunnion. Make sense? The rear trunnion/tang holes are the same way - countersunk.
Here is a pic of a normal "flat" neck rivet:
Here is a pic of a "swelled" neck rivet - although the ones we use on AK's are less complicated. Ones we use have the same round top as above with a full "countersink" bottom on the head. But this is the best pic I could find. You get the idea:
The raging debate is whether or not this countersinking or "dimpling" of these rivets/holes add any strength to the build. Or if it was originally just used to line up the holes in the receiver to the holes in the trunnions - when the went to line them up in their rivet machine. Meaning the Russians or whomever.
So the simple question is - does "countersinking" (or dimpling) rivets make a build stronger? And if so, by how much? Enough to go to the added trouble? Cause it's a b**ch to get the dimple in the receiver sheet metal. Or at least an added step. Most of the time it works but sometimes it doesn't. And your left with a hole that just won't seem to take a dimple. If you end up with a bad rivet head it's usually one caused by having to deal with the dimpling. Whereas a flat neck rivet goes in smooth every time. Alot of the later model Yugo's don't even use the dimple and swell neck rivets.
The area around the hole in the receiver sheet metal that is dimpled (or countersunk smashed) is very tiny. Less than a MM wide around the hole(?). IMHO countersinking swell neck rivets are not that necessary. Flat heads will work fine.