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RPK Part IV: maximum AK-47 Firepower: with the barrel set in the trunion, it's time to add the fixings to the barrel and complete the budget-priced build.

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Once you have the barrel set and pinned in place, the hardest part of the build is complete. Now the salvaged barrel attachments need to be installed on the new barrel. The pins to retain these parts are smaller than the barrel pin and the pinholes can be drilled using similar methods. If you are careful, you can drill the small pinholes accurately enough that you may be able to reuse the original pins. You just use the existing holes as drill guides and be careful not to wallow out the holes in the parts. If you do have to go larger, you can use drill bit shanks of number size drills for pin material. The pins need to be very tight so they don't work out.

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The front sight base, the rear sight base and the gas block are very tightly fitted to the barrel. The fit should be a light press fit of about .0005"-.001". The handguard retainer and the bipod are fitted loosely (.002"-.005" over barrel size) because they need to slide or rotate on the barrel. I started by installing the rear sight base. The base just pushes on till it is up tight against the trunion. It will fit between the upper sides of the trunion as long as you start it aligned correctly. Once you have the base positioned you need to pin it in place. I just used the existing hole in the base as a drill guide as I drilled the hole. Due to the fact that the base is already press fitted, the pin will not be under much stress and you can use a roll pin (also known as spring pins or tension pins) to retain the base rather than a solid pin. I can hear traditionalist AK builders cringe at this statement! A spring pin will tightly fit a hole over a broad size range and be much less critical on hole size.

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The next part to be fitted to the barrel is the handguard retainer. This part needs to be loosely fitted so that it can move forward and rearward during handguard removal and installation. A half-round slot in the top of the barrel will allow the retainer to lock in place. You have to cut this slot yourself if you made the barrel, not bought it. The slot can be cut by hand filing or you can use a ball end mill in a milling machine. You determine the correct position for the groove by test fitting your hand guard retainer and hand guards.

The gas block now needs to be installed, and this part must be fitted tightly to prevent gas leakage. The barrel journal should be at least .0005" over gas block hole size with .0015" being a better fit. This light press fit should be adequate even though it is much looser than a factory fit. The gas block is oddly shaped and it is hard to install with a very tight press fit without damaging the block. The position of the gas block is determined by the fit of the gas tube. You want the gas tube to fit as tight as possible yet still allow the tube to be easily removed for cleaning. In a factory setting, the right-to-left positioning of the gas block is determined with special fixtures you likely don't have. Every AK I have ever built featured a gas block aligned by eye alone. If you can see reasonably straight you should be able to get the gas block aligned well enough to work OK. This method is crude, but it does work. When pressing the gas block on to the barrel, use an improvised tool that is soft enough that it will not mangle the gas block as it is being pushed into position. I used a piece of aluminum pipe over the barrel that fit against the shoulder of the block. Rather than using a hydraulic press to push the block on, I used a small hammer to strike the aluminum tube. It looked crude but it worked fine and that's all that matters. After you have the block positioned pin it in place. I drilled carefully through my existing holes and re-used the small original cross pins.

Once the gas block is pinned in place, you will need to drill the gas port. To drill the gas port, you just use the existing passageway in the gas block as a drill guide. You will need a long 1/8" drill bit to drill the gas port so that you have enough length to set the barrel up in a drill press and still have enough drill bit to drill through the barrel. Because the drill bit will be drilling into the rounded side of the barrel, you will need to be very careful when you are starting the hole. You also must be very careful not to drill too deep and drill into the opposite side of the barrel once the bit passes through the barrel wall. A piece of 1/4" rod in the barrel will let you feel when the drill has passed through the barrel wall. When drilling the gas port, drill very slowly and use plenty of lube so that you drill a nice clean hole without burrs. If you do have a little burr on the inside edge of the gas port, use a small needle file through the hole to clean up the edges of the hole. While I set my gun up in a drill press to drill the hole, you can need a pretty odd set up to drill accurately. Many builders drill the hole with a hand drill, but you need to be extremely careful because you don't have very good angle control or depth control when using a hand drill.

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The bipod now just slips over the barrel because it needs to rotate slightly during usage. The bipod is retained by the front sight base. Drilling the pinholes is just like the rest of the holes but there is one important difference here: The front sight base must be properly aligned so that the rear sight can be centered when the gun is on target. On my gun projects I normally improvise my alignment method by bore sighting through the barrel and then aligning the sight picture to match the bore sight view. Unfortunately on this project, you cannot see through the bore due to receiver obstructions. You can buy pricey alignment fixtures, but those are rather costly for a single project. While it is easy enough to just look at the sight and make the sight look straight and centered, that is not accurate enough. I have an improvised method that will get the sights close but exact alignment will require actual shooting the gun and checking for good alignment.

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The method I used to get the sight aligned "close" was to clamp straight edges to the front and rear sights. With scales or pieces of straight scrap steel clamped to the flat sides of the front and rear sights I sighted and aligned the scales till they were parallel to each other. Once the two sights were parallel to each other I pinned the front sight in position. I preliminarily installed the front cross pin only. This pin, besides holding the sight in position, also retains the flash suppressor spring and detent. Remember to install the spring and detent before driving in the cross pin. Leave the rear cross pin hole for later when you have actually fired the gun and verified that the sight position is correct. There is no use in drilling two pin holes in the wrong place! If it takes you a couple tries to get correct sight alignment, you have the option of using larger pins when you reposition the sight base.

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After I had my sights aligned, I installed my YHM flash suppressor that I had rethreaded to match the special size threads that I had cut on the muzzle. The flash suppressor needs a small groove formed in the rear to interlock with the detent in the sight base. This can be cut with a milling machine or one can simply grind a groove with a Dremel tool.

After all barrel components were fitted I turned my attention to fitting the rest of the parts. I hand fitted the top cover, handguards, buttstock and pistol grip. I also did a couple other small jobs not completed earlier. I used a small drill bit to drill a couple dimples in the side of the receiver to act as safety lever detents. I test fitted the fire control group and checked for proper functioning. To retain the hammer and trigger pins, I purchased small pin retaining clips at the local hardware store. I had originally planned on using the I.O. Inc. retaining plate, but my small MIG welds used on the rails got in the way of the retainer. I also installed the rear sling attachment. This is simply a steel ring that is wrapped with a sheet metal strap that extends through the side of the receiver. Bending the strap over supposedly holds it in place but this looks like a poor retaining method to me.

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I then decide to test fit my I.O. Inc. scope mount to the riveted on rail. This is where I found a small problem with the scope mount. The locking mechanism was incorrectly made for some unknown reason. The locking tab extended above the bottom of the dovetail groove, and when tightened down it caused the mount to push away from the rail. This made the mount crooked and also prevented the, scope mount from returning to the same location every time it was removed and installed. I removed the locking tab and filed it down so that it was even with the bottom of the dovetail groove. Once I did this, the mount would tighten down tight against rail and securely stay in proper position.

After all this fabrication and fitting work, some experienced AK builders may be saying, "Steve, you forgot to heat treat the receiver?" No, I did not forget it, I did not want to do it and I will explain why. I did not do a complete receiver heat-treat nor did I even do spot heat treating of the hammer and trigger pinholes. For the hobbyist, who this article is written for, with little or no heat-treating skill or experience the process of heat treating an AK receiver is an invitation to disaster. The skill, knowledge, equipment and fixtures needed to heat treat an AK receiver without damaging it are typically beyond the abilities of the hobbyist. An improperly heat-treated receiver can come out as rippled as a potato chip if not done right. I have a fair amount of industrial heat treating experience and I would not ever consider heat treating an AK receiver at home because I do not have a temperature controlled heat treat oven or the fixtures to hold the receiver during heating and quenching. While I do spot heat treat thin 1mm (.040") receivers when I only occasionally use them, I feel that 1.5mm (.060") receivers are much stronger than 1mm receivers. Due to the extra thickness and bearing surface in the holes, I feel that heat-treating the receiver is not required unless one is going to expose their project to hard military usage. If you insist on heat-treating the receiver, I suggest you have a professional heat treater with AK receiver experience do the job. The S100 or more expense is better than ruining your project after you have done all this work. Your other option is to simply buy a competed receiver that was professionally heat treated by the manufacturer.

With all the parts fitted, it was time to see if my project would actually work. I started by doing some workbench function testing with dummy ammunition. Never, I repeat NEVER use live ammo for function testing. If you don't know how to make you own dummy ammo it can be purchased from Brownells. Function testing with dummy FMJ ammo revealed that the gun stripped rounds out of the magazine, chambered them, extracted them and finally ejected them forcefully. I tested functioning with several types and brands of magazines to verify that the gun worked right. AKs are very forgiving with their broad tolerances and tend to work well if you do a reasonable job of building them.

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While on the subject of function testing, I will give readers a little tip concerning the inexpensive Korean made 75-round drum magazines that are on the market today. I ordered two drum magazines for the RPK projects from CDNN. One was a military specification Romanian 75-round drum magazine ($99 on sale with a regular price of $129) and the other was a Korean commercial copy ($59) of the military drum. Obviously there would likely be some differences considering the price difference. The Korean copy was made from thinner steel and featured a painted finish instead of parkerizing. The Romanian drum worked flawlessly and felt very smooth as it stripped rounds out of the feed way of the drum. The Korean replica worked about 95% of the time but it operated nowhere near as smoothly and would hang up occasionally when rounds got stuck in the feed way. It appeared that one of the ways the Korean maker saved money on the drum-was not to deburr anything. There were several places in the feed way (the part on top of the drum section that looked like the top of a magazine) that had large burrs or sharp edges that scraped on the cartridges. Some of these were so bad that it was causing stoppages. To correct this problem I used some small files and sand paper (180-400 grit) to remove the burrs and smooth out the inside of the drums feed way. The main areas of concern were the feed lips, the front edge of the feed way where rounds slide up and out of the magazine, and the follower in the drum. These areas and any other place that contacted the cartridges were smoothed up to promote smooth feeding. The follower featured a couple slots that had a tendency to catch in the cartridges' extractor groove and prevent the last round from feeding. The edges of these slots were beveled just like on the military drum. Once all the problem areas were smoothed up, the drum worked as well as the expensive military drum. In this case a little work saved me $40. As long as you are willing to do a little tuning the inexpensive Korean drums can save you quite a few bucks.

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After successful workbench testing, it was time for live fire testing. When doing initial live fire testing load only a few rounds in the magazine. That way if you have done something wrong and the gun turns into a full auto you only have a few rounds going off rather than having an unexpected 30-round magazine dump. After you have verified that the fire control group operates correctly you can load more rounds into the magazine and continue testing. When doing function testing with new AK build only use full metal jacket ammo. AKs were never designed for soft point or hollow point ammo. Some AKs do not feed these types of sporting/hunting/commercial ammo well. You don't want to try to fix problems on your new project when the only issue was that you were using a type of ammo never made for the gun. Many AKs work fine with non-FMJ ammo but you want to do your testing with the ammo the gun was meant to use. At this point I did no accuracy testing or sight alignment since my only concern was functioning. Live fire testing revealed that my project worked fine. All rounds fed, fired, and ejected fine. Empty cases were thrown 8-12 feet away at the 2 o'clock position. AKs typically feature very forceful ejection.

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With the Yugo RPK working fine, it was time to apply a professional-grade finish to the project. At this point the project was a mix of old parts with little original finish, new parts with no finish and a mismatch of other rusty finishes. While the original finish was common bluing, I wanted something a little more durable that also required less work to apply. I also wanted a maintenance-free finish. I decide to use Lauer Custom Weaponry's DuraCoat firearms finish and Brownells GunKote firearms finish. Both of these are modern sprayed-on polymer finishes that are incredibly durable. Both finishes are available in many colors. I decide to use DuraCoat in the color of "Parker" on the barrel, receiver and bipod. I would use "matte black" Brownells GunKote on the top cover, gas tube, flash suppressor, and a few other small parts. I would also use some GunKote in the color of stainless steel for the bolt carrier. For those readers wanting in depth information on how to apply these great modern firearms finishes, I recommend you go to the store section of the Firearms News website and purchase (about $20) a publication called Shotgun News Gunsmithing Projects. This 400-plus-page book is filled with dozens of gunsmithing articles. The subjects of applying bluing, parkerizing, GunKote and DuraCoat are covered in several of the articles. The articles are written by many of the knowledgeable authors that have graced the pages of this magazine over the last 15 years.

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I applied these finishes in my workshop since they are very easy for the hobbyist to utilize. A brief explanation of the application process is as follows. First the parts are cleaned in solvents such as paint thinner or mineral spirits to remove all traces of oil, grease or dirt. Parts to be GunKoted are abrasive blasted to improve finish adhesion. I blast DuraCoat parts also but it is not required according to LCW. I then re-clean the parts in a strong solvent like lacquer thinner or acetone to remove any remaining oil or grease that may have worked out of the seams during blasting. The parts are then hung on hooks. After the liquid finishes are prepared, the parts are sprayed with a miniature automotive spray gun (available from Harbor Freight stores for about $20). After finish application, the parts are set aside to air dry for several hours. The GunKoted parts then are baked at 300 degrees for an hour to cure the finish. Once cool, the parts can be re-assembled since they will be at full hardness. The DuraCoated parts can be set aside for air-drying only but I bake them lightly at about 135-150 degrees to speed the drying and curing. DuraCoat both air-dries and also cures from the chemical reaction of a hardening agent that was previously added. Air drying will take several hours and full cure from the chemical reaction will take a few weeks although you will not likely notice the slow process of the chemical cure.

Both of these finishes are great finishes with specific attributes. If you have the required spray equipment, you can apply a professional-grade finish for as little as $35-$45. For those without spray equipment, DuraCoat is available in aerosol form although the aerosol is much more expensive that the DuraCoat that you spray with your own spray equipment. If you do good surface preparation and apply your finishes well, the resulting job will look as good as any factory finish. I am sold on these modern sprayed-on polymer finishes. It has been years since I have applied the old-time finishes such as bluing or Parkerizing due to the excellence of these modern "wonder finishes".

After my finishes were fully cured I re-assembled the Yugoslavian RPK. The completed projects looked and worked as well as any factory made gun at a significantly lower cost. My economy version of the RPK came in at about $520 (minus finishing cost) and that is a bargain price for a RPK in today's market. This low cost is several hundred dollars less than a typical used RPK in decent condition. Self-building you own guns is an excellent way to put guns into your collection that you may not be able to otherwise afford. I have vaults full of self-made guns that were made at a fraction of the cost for factory made guns.

In the next installment of this series, I will complete the premium RPK build. The project will feature all factorymade parts so it should appeal to those who prefer to let someone else to some of the project work. This project will feature some very interesting modifications and features that will let the gun run like it was originally designed to run. Until then why not consider giving hobby gunsmithing a try!

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Author:Matthews, Steven
Publication:Firearms News
Date:May 20, 2016
Words:3532
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