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Part 2: building a "Krinkov" style semi-auto AK-47 pistol.

Last time (2/10 issue), Matthews shortened a barrel and modified the gas system. This time, he forms the receiver.

Now that all the barrel work is done, it's time to fabricate the receiver. The receiver will be formed from a frame flat. There are many brands of frame flats, and each has its own proponents. Any discussion on AK-themed forums about which brand is best opens up a big can of worms. I am not advocating any specific brand of flat for this project. I used the TAPCO flat because even though TAPCO no longer offers it, it remains one of the most widely available flats.

Sources for frame flats can be found in SGN or by doing an internet search. Flat prices in late 2014 run $25 plus or minus $10. For the last several years, I have been making my own flats for less than $5 each. See SGN Treasury #14 for more info on how to make inexpensive frame flats.

They are available at most large gun shows and by mail order from various AK parts suppliers. The TAPCO semi-auto frame flat is made from 4130 alloy steel and is .040" thick. It has almost all the holes drilled and all cutouts are in place. It also features pre-bent top rails.

Side rails are included with the flat and have to be welded in place by the builder. This flat must be precisely bent to form an accurate receiver. While some builders just place the flat in a vise and beat it into shape, that's a very poor method to build a good AK receiver.

The best and easiest way to form an AK flat is with an AK flat bending jig. Purchasing a jig for a single project is cost prohibitive. The best method is to share the cost with other AK builders in your area or rent a jig.

Jigs are great if you are going to make several projects and can spread out the cost. There are plans for homemade jigs on the internet, but if you figure your labor to build it, you might as well just buy a factory made receiver.

I already had a flat bending jig, so I did not have to purchase one. I used the DPH Arms AK flat bending jig. It is priced at about $200. This cost can be reduced with a simple plan. Buy several frame flats and bend up as many as you think you may ever need, then sell the jig to other builders at a reduced cost. The DPH Arms flat bending jig comes with instructions, so I will only briefly cover the bending process.

While there are many companies offering bending fixtures they can also be easily self-made. In my SGN Treasury #14 AK article I show how you can build your own AK.frame flat bending fixture for about $25.

To use the DPH Arms jig, you place the flat over locating pins on the punch portion of the jig. A retaining plate is then installed to hold the flat in position on the punch. The punch is then pressed down into the die portion of the jig with a hydraulic press.

As the punch is being pressed into the die, the sides are bent up to form the receiver. The flat and die should be lubricated with grease before bending. Once the punch bottoms out in the die, it is removed from the die by tapping it out with a soft-faced punch and hammer.

Once the die is out, the sides of the punch are removed so the punch can clear the magazine well dimples and be removed from the bent receiver. When the bent receiver is removed from the punch, the sides may not be bent all the way to 90[degrees] because of spring back.

This is normal, since the stiff frame flat cannot be bent past 90[degrees] in this simple type of bending jig. A full 90[degrees] bend can only be done with a bend slightly past 90[degrees] when bending to account for spring back or by a process known in metal stamping operations as "coining" when forming.

To get the full 90[degrees] bend, just clamp a piece of steel to the receiver and bend it a little more by hand. Once you have a bent frame flat, it is considered by the BATFE to be a firearm receiver.

Although BATFE regulations do not require home builders to mark their firearms with maker's information, mark projects simply as a "cover your butt" policy. I know of one AK builder who found this out the hard way. He had built his semi-auto AK pistol and did not mark it. He was proud of his creation and took it to a gun show to show to his shooting buddies.

A law enforcement officer at the show took special notice of this rather sinister looking gun and requested to examine it. Since the officer was unfamiliar with AK pistols, he probably thought it was a rifle that someone had cut down, which is a felony.

He noticed no maker's name or serial number, so he thought this indicated that someone had removed the serial numbers as a way to make it untraceable, another crime.

He was also concerned that the gun may have been an illegal and unregistered full-auto, yet another offence! He thought he had caught a gun criminal and wanted to arrest the owner. The owner tried to explain the laws surrounding the home building of guns, but the officer felt he knew more about law than the owner, which he didn't.

The owner finally convinced the officer that the gun was actually made as a legal homemade semi-auto pistol by taking him to a reputable vendor's table that specialized in AK building and having the vendor explain the laws.

You might argue the owner was right and the officer was wrong, but getting arrested and having to go to court to prove your innocence is rather inconvenient! Innocent until proven guilty looks good on paper, but in the real world it won't keep you from getting arrested if some uninformed law officer thinks you have done something illegal. Simply stamping the receiver with the manufacturer's name, address, serial number and a designation as an "AK Semi-auto pistol" would have prevented a lot of problems. An inexpensive set of metal stamps costs about $10, which is cheap insurance against this type of misunderstanding.

With the frame bent, the front and rear trunnions can be installed. The front trunnion slides into the front of the receiver with the top rails sliding into grooves in the trunnion.

If you bent your flat accurately, the holes in the receiver should align with the holes in the trunnion. Some hand fitting of the top rails may be needed to get the receiver sides flush against the trunnion.

The front trunnion is secured to the receiver with rivets. AK rivets can be purchased from AK parts suppliers, but I have a selection of bulk rivets in my workshop, so I used them and did not buy my rivets. Installing rivets with basic hand tools is very easy but rather slow. Unless you're going to be doing a lot of riveting, rivet setting tools are not needed. I have always set rivets with nothing more than hammers, punches, and a selection of steel backing blocks to keep cost down.

The first rivets to be installed are the two lower rear rivets. These rivets were set by supporting the heads on the inside of the receiver with backing blocks and then forming the heads on the outside. About 3/16" of shank extension out of the side of the receiver will give enough material for head forming.

Be absolutely sure when forming rivet heads that the opposite head is solidly supported with steel backing blocks. All the hammering forces used to form the head need to go into the rivet only and not the surrounding parts. Failure to properly support the rivets when setting them can damage the trunnion or receiver.

To set the rivets, I simply flattened the heads with a small hammer while the parts were clamped tightly together. Careful forming with a hammer can yield nicelooking heads. Once the heads are flattened slightly, you can tap around the edges to form a rounded head.

You can chose how you want the heads to look. You can make them rounded or flat or a combination of the two. When forming rivet heads, use a lot of small hammer strikes, not a few heavy hits. As you are forming your head, check every so often that the parts are still tightly together.

You don't want to find unsightly gaps when you think you are done. If you have gaps you will have to do it over; rivets must fit tight.

The two rivets in each side of the trunnion need to be installed with the aid of a round backing block in the barrel hole. You can make a backing block on a lathe or you can use the old barrel stub as a backing block. Either one has to fit very tightly in the barrel hole.

When the tool is slipped into the barrel hole, you want it tight with no play. It doesn't have to be tight like an installed barrel, but it must be snug. Install the rivets in their holes with the heads on the inside of the barrel hole. Be sure they are seated completely against the trunnion.

You may need to trim the heads slightly to allow the backing tool to enter the hole. Do not over-trim, you want the heads to be tight against the backing tool when it is installed in the hole.

You need about 3/16" to 1/4" of rivet shank extending out the side to form a rivet head, depending on the size of head you desire.

Lay the assembly on a thick piece of steel plate. Locate it near the edge so a C-clamp that is securing the parts can hang over the edge and not interfere with riveting.

You can set the rivets one side at a time or with all rivets installed in the trunnion and receiver. If you have rivets in both sides, you will need to drill a couple clearance holes in the steel plate so that the rivet shanks don't keep the receiver from lying flat on the steel plate.

With the receiver flat against the plate, use a small hammer to form the rivet heads to whatever shape you want. Be absolutely sure that the parts are tightly clamped together with the C-clamp when setting these rivets. They must be tight with no gaps between the parts or under the rivet head.

Set the rivets on both sides, then remove the round backing bar and grind a little off the heads inside the barrel hole to be sure there will be plenty of clearance when the barrel is installed.

Before the rear trunnion can be installed, it should be modified into a pistol trunnion. The rear trunnion should be modified so it cannot accept a buttstock. This is to meet BATFE guidelines. If an unmodified rifle type trunnion was used the BATFE could consider the firearm as being "readily convertible" to prohibited short barrel rifle configuration.

To turn the trunnion into one that cannot accept a buttstock, fill the stock mounting recess in with a steel block welded in place. This makes the trunnion completely flat on the rear.

I placed a 10-24 threaded hole in the center for a sling swivel. The rear trunnion slides into place on the rear of the receiver with the top rails fitting in grooves in the trunnion. Some fitting may be required for a proper fit.

The TAPCO receiver flat was made for a fixed stock rear trunnion, and the holes are not completely correct for a folding-stock AMD-65 trunnion. One set of holes will be correct but the other set will need to be relocated and drilled.

The rear rivets are long and extend all the way through the trunnion and receiver. About 3/16" extension on the rivet shanks are about right for head forming. Securely clamp the parts together, lay the receiver on a steel backing plate and set the rivets. Once the front and rear trunnions are installed, trim the excess receiver so it is flush with the trunnions.

Installation of the trunnions has stiffened up the flimsy sheet metal receiver. Now is the time to do a rather touchy operation, heat-treating part of the receiver. There are differing opinions on how this should be done. Some insist that the whole receiver should be heat-treated. Others think only the fire control pin holes need heat-treatment. Both sides of this issue have their good reasons for doing it their way.

It is my amateur gunsmith's opinion that the fire control pin method is best for the home builder. Complete receiver heat-treating with improvised and inaccurate methods and equipment is a recipe for disaster.

The receiver can be warped so bad that it is ruined if the whole receiver heat-treat is done improperly. The fire control pin only heat-treat method is very easy to do and only takes a few minutes to do, but it must be done correctly.

You will need an oxy-acetylene torch with a small diameter tip (about 1/4" wide flame). A common propane torch will not work for this purpose. The flame is too wide and the flame does not have enough heat quickly to heat up the holes without heating the surrounding metal.

I used this method and it works well. Variations of this method are commonly used in industry for what is known as spot heat-treating, so it is a proven method. Besides the torch, you will need a piece of sheet metal to act as a heat shield. Lay the receiver on a flameproof surface with one set of holes up.

Place your sheet metal heat shield in the interior of the receiver at a 45[degrees] angle so that it will direct the flame away from the interior of the receiver when the holes are heated. Set your torch for the hottest neutral flame.

Place the narrow flame into the hole so that the majority of it passes through the hole and only the very edges of the hole are heated. You must quickly heat the very edges of the hole without heating the surrounding metal.

The thin receiver material means this happens very quickly, only a couple seconds. The hole edges need to be quickly heated to an orange color (about 1500[degrees]). As soon as the edges are heated to the right temperature, immediately remove the torch.

If this is done right, the cool surrounding sheet metal will cool or quench the hot portion fast enough for heat-treating/hardening. If the surrounding material is too hot, it will not cool the hole edges fast enough to set the steel's internal structure in a hardened state.

This procedure must be done very quickly and accurately. You have to hit the hole just right on the first attempt or you risk melting or warping the receiver. I highly recommend practicing on a piece of scrap sheet steel the same thickness as the receiver before you try doing the receiver.

This heat-treating method is very easy to do but it is also very easy to mess up. Do only one hole at a time and allow the receiver to cool before you do the next hole.

The 4130 steel of the receiver will not get as hard as some tool steels when heat-treated, but it will be considerably harder than in its soft state.

To verify the hardening of the hole edges, use a small round file and take a light cut where it won't show on some flat part of the receiver. Then use the same file and take a light cut (don't cut much!) on a hole edge. You should notice that the hole edge is noticeably harder than the rest of the receiver.

The rails that came with the frame flat will need to be installed on the insides of the receiver. The ejector is formed in the front of the left rail. It must be heat-treated for wear resistance. Heat-treating will only need to be done on the ejector tip.

Using a torch, heat the point orange hot (about 1500[degrees]). As soon as it is hot, immediately quench the rail in mo tor oil. After cooling, check for hardening by doing the file test.

Before the rails can be welded in the receiver, you need to check for bolt carrier fit. The carrier has grooves that ride on the upper rails of the receiver. The carrier must slide on these rails smoothly without excessive clearance or tightness.

During receiver forming or heat-treating, the sides of the receiver may have bowed slightly. Verify that the sides are straight; if they're not straight, correct them.

If the sides are straight and the carrier is too tight, you will have file a little off the rails for easy operation. While the carrier rides on the upper receiver rails, the bolt itself will ride on the lower rails.

The lower rails must be positioned correctly so that the top edge of the rail is even with the edge of the bolt groove in the front trunnion. This is so the bolt will transition smoothly when it goes from rail support to trunnion support.

The lower rail must also be parallel to the top rail. The easiest way to get it straight is to lay a proper size drill bit between the rails to maintain proper spacing while you clamp it in place. Holes in the lower rails are supposed to align with the cross-member rivet holes in the receiver.

These may align, but likely will not. Do not be concerned if they don't. Rail alignment with the trunnion is the most important issue.

My method to locate the rails for welding is somewhat different than most, but it does work well. With the rails located and clamped in place, drill two 1/8" holes through the rails and receiver. Locate these holes away from the heat-treated holes.

I then used pop rivets to secure the rails to the receiver. Before any one gets alarmed or starts laughing, let me assure you the pop rivets are for temporary use only. With the rails secured with the pop rivets the clamps can be removed and the bolt and carrier can be installed and checked for proper operation. If the rails are located right, they can be welded in place.

There are a couple methods to secure the rails to the receiver. Some who don't have welders use solid rivets to attach them, but the most common method is to weld them in place with a MIG or resistance spot welder. Home workshop grade spot welders are available but this adds cost to the project, since most don't have spot welders.

I also question the quality of welds these light duty units can make. I set up, repaired and operated industrial spot welders for 20 years in my previous job as an industrial machine repairman. Industrial spot welders that would be sized right for this operation make spots about 5/16" in diameter, have over 400 lbs. of tip pressure, and generate 7000-10,000 amps of weld current.

The home workshop grade welders make spots only about 1/8" diameter with extremely low tip pressures and only 1000 to 1500 amps welding current.

While many report good results with these light duty spot welders, my years of experience make me skeptical. I therefore MIG welded my rails in place. With the rails correctly positioned and secured with the pop rivets, I placed a small weld at each end of the rails where it wouldn't interfere with operation of the bolt and carrier.

Once the rails were secured, I drilled out the pop rivets and drilled out the 1/8" holes to 3/16". These holes were then plug welded to fuse the rails and receiver together. A block of aluminum or copper loosely fitted behind the holes will assist in making good plug welds.

I ground the excess weld flush. After rail welding, recheck bolt and carrier operation, since weld heat could have bowed the receiver slightly.

Now that the rails are installed, the center support can be installed in the interior of the receiver. This support stiffens up the thin sheet metal AK receiver, and is nothing more than a long rivet that extends through the sides of the receiver and a thin tubular spacer.

The spacer is sized to the receiver's internal width. These can be purchased, but I made a perfectly good one out of a long rivet and a roll/spring pin. The homemade version only cost a few cents. Just buy a roll pin that allows the rivet to pass through the center.

If the roll pin is a little loose, just squeeze it down so it fits the rivet tight. Trim the roll pin to the receiver's internal width and install it with the rivet.

The holes in the receiver wall probably don't align perfectly with the holes in the rails, but this isn't a big issue. Just file the hole in the rail to match the hole in the receiver. If you want the center support even more solidly secured to the frame, place a small MIG weld on each end.

Next time (4!10 issue): Finishing the pistol

Ed. Note: Reid Coffield is continuing his recovery from a fall during the summer and will continue his series on building a Springfield 1922 rifle in the 6110 issue. In the meantime, Steven Matthews is capably filling in.
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Author:Matthews, Steven
Publication:Shotgun News
Date:Mar 10, 2015
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