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AK-HD building a heavy duty semi-auto AK-47 receiver part 2: in part 1 (1/20 issue), Matthews built a frame bending jig and bent his extra-thick AK receiver. This time, he forms the rails, installs the trunnions and trigger assembly and finishes the receiver.

Now that the bottom of the flat is bent, the top rails need to be formed. An easy-to-make fixture will be used to bend these rails at the correct height. It will be made from a 101/4-inch piece of lx1 1/4-inch bar stock. This fixture will be placed in the interior of the receiver and the top rails gradually formed over it with a hammer.

I welded small steel blocks to its ends to angle it to create the correct front and rear inside dimensions. The AK receiver needs to be 13/1h (1.187) inches at the front and 1% (1.625) inches at the rear. These are inside dimensions, not outside measurements.

Weld a 3/16" block to the front end of the 1 .25-inch wide bar and a 5/8" block on the rear of the bar. Bevel the 5/8" block towards the center to angle the fixture and maintain the desired 1 5/8-inch size on the very end. The sides of the blocks will need to be radiused slightly so the fixture can be flush against the bottom of the receiver.

Place the fixture in the receiver, making sure it is tight against the bottom. Clamp the receiver and fixture in a large vise so that the edge of the fixture inside the receiver is flush or just slightly above the edges of the vise jaws. You may want to use some scrap stock between the jaws and receiver to prevent marring the sides of the receiver.

Once this assembly is very tightly clamped in the vise. you can start bending the top of the receiver over to form the top rails. Use light taps and gradually bend the edge over. Only bend where the part is securely held by the vise jaws. Bend the material over about halfway and then reposition your work in the vise to support solidly the remainder not yet bent.

Failure to support the sides in the vise while bending will result in poor bends. Continue to slowly form the top rails. Bend partway, then reposition and bend some more. Gradually bending the rails will give better results than trying to form them with just a few heavy hits of the hammer.

Make sure every time you move your work that the fixture is still tight against the bottom of the receiver. Be sure that the rails are being bent flat and tight against the fixture. Any bumps or dimples need to be worked out before removing the fixture. If you have formed the rails right, they should be tight against the fixture with good 900 bends.

To ease bending of the top rails. I left them greatly over finished size. The size can now be reduced. Narrow the top rails on each side to an internal width of 9/64"-5/32". This width will still leave some extra material for final fitting. The rails can be easily cut to size by scribing a line on. the top and then cutting with a hacksaw. File the rough cuts smooth.

To fit the grooves in the bolt carrier and trunnions, the top rails must be thinned down from their original thickness of .071". I thinned mine down to .055". This was about 30% thicker than a standard .040" thick receiver and should result in much stronger top rails. The rails can be thinned with a milling machine, file, sander, grinder, etc. Just be sure you thin them evenly without high and low spots.

If the trunnion you are going to install in this receiver still has the barrel (or a barrel stub) installed, it must be removed to install the trunnion in the receiver. The barrel is removed by pressing out the crosspin and then pressing out the barrel. This often requires a hydraulic press with considerable tonnage. Be careful not to damage the trunnion or barrel when you remove them.

The trunnion can now be installed in the heavyweight receiver. I will only briefly cover this, as I have written about it before (9/20/05). The grooves in the trunnion may or may not accept the thicker top rails, depending on how they were cut. The grooves can easily be widened with a thin abrasive cutoff wheel in a small power hand grinder or Dremel tool.

You will also have to broaden the radiuses on the sides of the trunnion to match the wider radius of the heavy gauge frame flat. The top rails in the front of the receiver will need to be narrowed down so that they can slide into the grooves in the trunnion.

Narrow the rails down, making sure the rails fit in the trunnion grooves and the sides of the receiver are flush against the trunnion. This is vitally important since the sides must be tight against the trunnion when rivets are installed.

Once fitted together, the trunnion and receiver can be riveted together. You will need a mandrel to back up the rivet heads on the inside of the barrel hole. This can be self-made or you can also use an old barrel stub that has been worked down to slide easily but snugly into the barrel hole.

Since this receiver is almost twice as thick as a stan: dard receiver I do not know if purchased AK rivets will be long enough for the project. Actually I have never used purchased metric AK rivet sets for any AK build I have ever done. Rather than buy a handful of expensive ($7-$15) metric rivets. I have always used standard size rivets that I purchased in bulk boxes. Holes in the parts were just enlarged to fit the slightly larger standard size rivets.

Standard size rivets are available from McMaster-Carr Industrial Supply Co. for literally pennies apiece. I bought one box each of 1/8", 5/32" and 3/16"x1/2" rivets and one box of 3/16"x2-inch rivets for roughly $4 per box Each box contains hundreds of rivets. This will cover a lifetime of AK building for the average hobbyist for the cost of one metric AK rivet set. I also take the cheap way out for setting rivets. Rather than buying expensive rivet setting tools. I set my rivets with nothing more than a hammer, punches and backup blocks. It is slow but it costs virtually nothing.

Install the trunnion in the receiver and install one 3/16" rivet (remember to open up your holes if using standard size rivets). Trim the head so it is flush against the inside of the trunnion.

Install your mandrel and observe how much rivet head needs to be trimmed to allow the mandrel to pass over the head. Trim the head to allow the mandrel just to slide over: the mandrel needs to be tight against the head to support it fully during rivet setting.

Trim the portion of rivet that is extending out of the side of the receiver to about 3/16" extension. This will leave enough material to form rivet heads as shown in the article pictures. If you prefer a larger, more rounded rivet head, leave more rivet extending.

Clamp the trunnion and receiver tightly together with a small C-clamp and then place the assembly on a hard, flat surface, preferably a heavy steel plate. Make sure the mandrel is supporting the rivet and then start forming the rivet head. 1 start by slightly flattening the exposed rivet shank and then use a punch with a hemisphere ground in the face to form a slightly rounded head.

The rivet head can be easily shaped by using light hammer taps around the edges to form the heads to the desired contour. During all rivet setting, be sure the parts are staying tightly clamped together and that the rivet heads are fully supported. Failure to observe this advice will lead to a poor rivet job.

Once the rivet head is tight against the receiver, you can go on to the next rivet and use the same methods to set the rest. The lower rivets in the trunnion can now be installed. They can be backed up by using bars and blocks through the magazine well.

Now install the rear trunnion. The upper rails will have to be trimmed and the trunnion will have to be modified just as the front was to accept the thicker receiver. Remember the trunnion must fit tight against the receiver wall.

The receiver and trunnion were drilled to accept 3/16" rivets that extend all the way through. I used 2-inch long rivets and trimmed them for about 3/16" extension. The clamped assembly was placed on a steel plate and the rivet heads were formed.

The existing rivet heads that came on my purchased rivets were larger than I like, so I ground them down to look more like my hand-set heads. Some like big bulging rivet heads, but I prefer a smoother, low-profile rivet head on my projects.

After the rear trunnion is set in place, the rear of the receiver can be trimmed flush with the buttstock. Be aware that wood fit on AK rifles can be pretty poor, and even your best attempts can result in a less than appealing fit.

The top rails now need to be fitted to the bolt carrier. The first thing that needs to be done is to remove completely a portion of the rail directly in front of the rear trunnion so that the rear of the bolt carrier can drop down into the receiver.

Only remove enough to allow carrier installation and removal when the carrier is all the way rearward. The remainder of the rails now need to be trimmed so that the carrier can slide on the rails. Trim the rails just enough to allow the grooves in the carrier to slide over the rails easily with no drag. Be sure to trim the rails on each side evenly so the carrier is centered laterally in the receiver.

Once you have the carrier sliding over the rails smoothly, you will need to cut shallow relief notches in each side to allow bolt installation. The bolt is slightly wider than the existing top rails and a little material needs to be removed so the bolt lugs can enter the receiver and ride on the lower rails that will be installed later.

The right and left bolt lugs are sized differently, so different sized notches will be made in each rail. To mark the notch location, just lay the bolt carrier with the bolt installed (bolt all the way forward) on top of the receiver. Locate the carrier all the way rearward in its removal position.

Mark cutout locations around the bolt lugs. Size them very close to the lugs; you only need enough clearance to allow the bolt to drop down into the receiver when the bolt and carrier are all the way rearward. Remove only enough metal to allow the bolt to enter the receiver.

While the bolt carrier rides on the upper rails, the bolt itself rides on lower rails that need to be installed. I used the rails that came with my TAPCO frame flat, but rails are available from AK parts suppliers that advertise in SGN such as DPH Arms, K-Var or Red Star Arms. Before the lower rails can be installed, the left rail must have the ejector formed into its side and must be heat-treated.

The whole rail does not need heat-treating, only the ejector itself. Heat the front edge of the ejector with a torch till it is bright orange and then dip it in oil while it is still orange hot. This will harden the 4130 steel.

Verify that it has been hardened by testing with a file. The ejector should be noticeably harder than the rest of the part. If you heat-treat only the front edge of the ejector, this will be adequate, since the rear of the ejector will remain softer and not brittle. If you heat the whole ejector before quenching in oil you may need to anneal the rear part to reduce brittleness.

As long as we are on the subject of heat-treating. I might as well bring up the hotly disputed issue of heat-treating the receiver. Manufacturers and home builders have many conflicting beliefs about how an AK receiver should be heat-treated. Some insist that the whole receiver needs to be heat-treated, while others prefer to only heat-treat the hammer and trigger pin holes.

It is my opinion that the extra heavy material used to make this receiver means that it is sufficiently strong and durable as is without any heat-treating. The hammer and trigger pins have almost twice the bearing surface of .040" receivers and I think it is more than adequate. If, however, you disagree with my opinion then feel free to heat-treat the project in any way you want, it's your project.

In a factory setting, these rails are usually resistance spot-welded in place. Most hobbyists don't have spot welders, and it just drives up project cost to buy one. An alternative method to install the rails is to rivet them in place with 1/8" solid steel rivets.

These lower rails need to be positioned even with the bolt lug grooves in the trunnion and parallel to the top rails. The bolt lugs need to transition smoothly from rail support to trunnion support when the bolt is reciprocating. You don't want any "bumps" when the lugs travel between the rails and trunnion.

To position the lower rails parallel to the upper rails use a drill bit as a spacer. Use whatever size bit that aligns the rail with the trunnion. Once the rail is positioned, clamp it in place with a couple small C-clamps. To make installation easier and to allow for checking before final installation, the rails will be temporarily secured with pop rivets.

Drill a 1/8" hole through the receiver wall and the clamped-in-place lower rail at each end of the rail. Be sure to drill in the U-shaped space under the upper edge of the lower rail. Install both rails with the pop rivets.

Once they're installed, remove the C-clamps and check for smooth operation of the bolt and carrier. If the rails are aligned properly, replace the pop rivets one at a time with solid 1/8" steel rivets. Be sure the rails are tight against the receiver wall when setting the rivets and be sure the rivets are tight.

I recommend three rivets in each rail unless you do as I did. I used two rivets in each rail but also added a couple small MIG welds on the inside of the receiver since I have a MIG welder.

Before you install the center support, you may choose to install the trigger guard/magazine catch first since the center support gets in the way of setting the trigger guard rivets. The trigger guard/magazine catch assembly features a thin plate between the trigger guard and receiver. This plate serves as a safety lever stop and also a stop for the lug on the rear of magazines.

Because of the extra thick material used for this receiver, this plate must be thinned by 1/32" (.032") so that the magazine will extend into the interior of the receiver to the correct depth.

Thin this plate by removing 1/32" from the side that faces the receiver. Removing material from this side will leave the spacing at the magazine catch unaffected.

The AK trigger guard is attached with five 4mm rivets. Since 4rrmi rivets are sized about .160", I replaced them with inexpensive 5/32" (.156") standard size rivets. Install the trigger guard and spacer, followed by four rivets in the holes. The preformed heads should be on the outside. Trim the rivets for about 1/8" extension into the interior of the receiver.

Back up the rivet heads with steel blocks and form the heads on the inside of the receiver. Be sure the rivets and trigger guard are tight during installation. The interior heads do not need to be shaped any particular way, since they don't show, I just smash them flat. Install the rear single rivet the same way.

Once the trigger guard was installed, the magazine well was final sized for good magazine fit. AK magazines can vary considerably, so you may want to test with several varieties. File the magazine well opening to allow for easy magazine installation without excessive clearance. When sizing the magazine opening, you will quickly notice that the magazine cannot be inserted all the way, as it strikes the lower rails. These rails need to be trimmed up to allow the magazine to seat fully.

Careful fitting of the magazine opening and the rails will result in a nice-fitting magazine with little wobble. At this point, some may ask about the dimples used on thinner AK receivers that serve as magazine stabilizers. This heavyweight receiver is too thick to dimple with home workshop tooling. If you carefully fit the rails and magazine opening you may not need these stabilizers.

If you do wish to have some type of extra stabilization between the magazine opening and rails, a thin piece of steel can be riveted or welded in place. I added these thin pieces to my project because I took a little too much off the rails and opening, which caused a slightly loose magazine fit. These added stabilizers removed the excess play in the magazine well.

With magazine fit taken care of, you can install the frame center support. My .073" thick heavy-duty receiver was so stiff and strong I don't know if the support is even needed, but I installed it anyway. This frame support is simply a hollow tube with a rivet through the center.

For the support I just drilled a 5/32" hole through a piece of 1/4" rod. It was sized to the receiver's internal width. Rather than buy a long 5/32" rivet, I simply made one from a large nail. The head was reformed and the diameter reduced slightly. There is little difference in material between a nail and a soft steel rivet, so even though it may sound crude, this will work fine.

The holes for the support rivet are already in the receiver, but you may have to drill the holes through the installed rails.

While the receiver is pretty well done at this point, there are a few more items to clean up. The extra thick receiver means the standard trigger and hammer pins will probably not be long enough. Extra-long pins can be obtained from Marsh Hawk Arms ( for a reasonable cost. I, however, decided to make my own from material I had on hand. To retain the pins I used hairpin type retainers that are available from well-stocked hardware stores.

The thicker receiver means the safety lever will need to be modified. Overall, my safety lever was long enough if it was thinned somewhat on the lever side. The flange that bears on the right side of the receiver was thinned and reshaped slightly so that the lever could extend further into the receiver. Once this was done, the end of the safety lever reached almost all the way through the left side of the receiver.

After I installed the safety lever. I made a small dimple in the side of the receiver to act as a safety lever detent. This will serve to lock the lever in the off-safe position during use.

One more thing needs done to the receiver; it needs to be marked. Although not presently required by federal law for homemade firearms, identification information should be marked on the receiver.

I marked mine with a serial number, maker's address, and a designation of semi-auto AK rifle. With a set of $10 hand stamps you can put any number or letter you want on your project. If you place your letters and numbers straight, it will look like a professionally marked receiver.

At this point the heavy-duty AK receiver was just about done. Some final fitting may be required when all your parts are installed. While the project might look a little ragged at this point, final finishing will dramatically improve its appearance. Removing any scratches and gouges can be done when the project is assembled on to a parts kit or now when the receiver is finished.

When I assembled my receiver on a Romanian "G" parts set, I did all finishing work. Once all scratches were removed and rivet heads cleaned up the receiver and complete project was abrasive blasted. This really evened out the finish. I then applied Brownells GunKote firearms finish in matte black. GunKote is a sprayed on and baked finish that is extremely durable. It is available in many colors and is easy for the hobbyist to apply. I can highly recommend BrowneIls GunKote.

When completely finished and assembled, the project looked as good as any factory-built AK-47. In fact it probably looked better, since most AKs are not finished well. From the exterior there was no telling this project had a heavy-duty receiver. It was however, almost twice as thick and vastly stiffer and stronger than the thin .040" thick standard receivers. If you like building your projects much stronger than minimum standards, this project may be worthy of your consideration. If this sounds like your kind of project then why not give it a try?
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Author:Matthews, Steven
Publication:Shotgun News
Date:Feb 20, 2013
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