Ah, the holy grail of AK assembly - finding the right rivet!
The factory-specs for front trunion rivets call for 4.5mm swell-neck rivets and 4mm standard rivets.
Here are some rivet schematics (drawn up from Plinker) to illustrate the basic swell-neck design:
Can't find 4.5mm swell-neck rivets? No problem! There are several ways to obtain these rivets...
No matter what you do, you can get buy with normal rivets but the swell-neck rivets are intended to be used. There is a great deal of information on the web about rivet design, and I haven't had a chance to compile this information here, but it is sufficient to say that the AK trunions are designed to used swell-neck rivets.
Here is a pic from K-Var that shows a cross-section of the swell-neck rivet assembly:
Here is a receiver section that shows how the swell neck dimple is formed by the rivet squeezing process. Note that this receiver was squeezed using my bolt cutter rivet squeezers. Note also that removing the rivets caused the edge of the holes to gall slightly...
Some (like Xebec) have been able to simulate the swell-neck by using a normal "mushroom" rivet, and wrapping piece of wire such as welding wire, around the shaft. This fills in the area normally filled by the swell-neck. Obviously, it is "best" to use the factory-style swell neck rivets, but in all honesty, the amount of usage that normal builds will endure should not lead to premature rivet failure. If the rivets begin to fail, one could always remove the barrel and re-rivet. :)
Rivet assembly methods are not very well known compared to say, screw assembly methods.
Here is a good "textbook" rivet information originally found by Vulcan762:
The following PDF doc is the Official Army machinist document describing bucking rivet assembly
So there you go, all you wanted to know about riveting. So, how does this apply to building an AK? Glad you asked...
In solid riveting, we are pressing the rivets hard enough to smash them into shape. There are several methods to supply the pressure for proper rivet compression, which we will explore:
This is probably the most expensive approach, but the use of a press yields the most predictable results. How much clamping power is needed to crush the rivets? Some have reported 3500 lbs. with pneumatic squeezers. Other reports have said that 6-ton presses are adequate. Most people choose a 12-ton press such as this one from HarborFreight due to the price point:
This method is somewhat of a deviation from standard riveting, because you don't apply pressure to the rivet head, but rather the bottom of the shank, just the opposite of the conventional riveting method. That being said, understand that this requires a custom-built jig that allows you to direct the force to the rear-most portion of the rivet shank. This is the part of the rivet shank that is INSIDE the trunion, not the head of the rivet you see from the outside of the rifle:
Plinker has created an excellent jig for just this purpose - he makes these when he has time and I think he sells them for about $100. He has graciously provided the plans for his jig assembly free of charge for those who have a mill or a really good drill press and access to the raw materials. Plinker's plans are listed here
I have saved these plans here:
This method, used with Plinker's jig, has been the most successful for those on the BIY forums.
Custom Press Variations
Ezra Coli assembled his rivet jig and besides the soft metal stressing from use, it worked for him:
This method uses a standard air hammer and is the most common rivet method used in other industries. The air tool method is the most hazardous for the novice, because if you slip with the air hammer, the face of the hammer will dent or "dimple" the receiver and leave a crescent-shaped mark that is difficult to remove. As always, practice on a test piece before attempting this on the final product.
You will need to locate a bucking bar that peens against the bottom of the rivet shank where the forming occurs. You also need to rent/buy an air compressor to power this tool.
Pneumatic Rivet Squeezer
This uses an expensive "alligator" rivet squeezer. The squeezer is cost-prohibitive and isn't widely used. These squeezers are usually around $300. You need an air compressor for this tool.
Hand Rivet Squeezer
This uses the same yoke as the pneumatic rivet squeezer, but with large handles like you find on a bolt cutter. These haven't been tested yet but they are rated to work for up to 3/16" rivets so they should work. Average price is around $130. The yoke would be inserted into the trunion and the handle side would brace against the head of the rivet. Some die would probably be needed to press against the rear of the rivet.
The cheapest method is the use of common hand tools. These methods do work and some are admittedly low-tech. YMMV.
Several people have reported good success in using an old punch. You need to dremel (or mill) a concave dimple into the end of the punch that seats against the head of the rivet. Use a bucking bar against the base of the rivet, and a large hammer (3-4 lb. sledge) to drive the rivet into position. This is identical to the use of an air hammer, save that you provide force with your hands in place of the air compressor.
While not recommended, you can use a scrap barrel section from K-Var as a bucking bar in a pinch.
Modified C-Clamp or Bolt Cutters
A poor-man's imitation of the hand rivet squeezer. Cheap and effective. Start with 24" bolt cutters ($15 from HarborFreight) and grind the cutters down into a clamp that imitates the yoke of a pneumatic rivet squeezer. Crude but effective. This photo shows a ground-down bolt cutter in the squeezing position:
This method worked for me, YMMV. Grind slowly, don't heat the metal and damage the temper of the steel.
Here is the modified bolt cutter assembly doing its job:
More info is on my boltcutter page
Here's a close-up pic of the jaws I modified: