If you look at the factory installed rivets (like in the Left side plate), their countersunk heads actually sit a little below the inside surface of the Side plate. These appear to have been set with industrial rivet setters that squeezed from both sides. Most home builders don’t have one of those lying around. The most common home shop technique is to carefully countersink the side plates so the rivet heads sit exactly flush, then clamp in the bucking bar and peen away.I don't have my parts yet, but I am reading tutorials to get a handle on things. My plan is to build from a 100% KMP plate. My rivet experience is confined to one AK build.
If the rivets are countersunk on the inside so that the rivet head sits a little below the surface, what keeps the rivet from being loose? Does the rivet head flatten up against the bucking bar and shank metal expand to fill the gap? Hopefully that makes sense.