RPK: Part II: maximum AK-47 firepower: in this installment, Matthews makes receivers and barrels for his RPKs.
In part one of this series, I sourced project parts and salvaged the parts I would need from the RPK parts kits. Before I can begin assembling the economy version of this project, two major cost-cutting procedures need to be done: a barrel and a receiver need to be fabricated. In order to save almost $200, (1) will turn a basic Green Mountain barrel blank into an RPK barrel. It will take several hours of basic lathe operation, but the cost savings are so substantial that it is well worth the effort.
The lathe work requires turning the barrel blank to sections of specific diameter and length. The diameter tolerance range will be one-half to one-thousandth of an inch, so any well-maintained lathe should be able to handle the job. Since the lathe work is basic, I will not go into much detail of how to form the barrel. I will, however, cover the high points and give many basic dimensions. The dimensions listed here are what worked on my specific project and you may need to fine tune the dimensions to fit your project. With that said, let's get started.
I had three sources for the dimensions I would use to fabricate my RPK barrel. First, I had the old pieces of barrel that my barrel components were mounted upon. When you salvage your barrel parts, save the old pieces in case you decide to fit your new parts as tightly as the original parts. You may, however, decide not to make the fit as tight as original since you do not have all the tools and fixtures that the factory used for extremely tight fitting. My next dimension source was a basic RPK barrel print that was listed on the Green Mountain website as an alternate picture of the featured barrel. Lastly, I had the new Green Mountain RPK barrel that I had obtained for the premium project. One thing you need to be aware of is that Yugo AKs feature parts that are considerably different than common Euro-format AKs. Also, remember that the tolerances for AKs are very broad and part sizes can vary widely even when using parts of the same manufacturer. Any dimensions obtained from anything other than your old parts are to only be considered as general guidelines not carved-in-stone specifications.
The Green Mountain barrel blank was vastly oversize at 1.375" diameter by 24" long. This means that a huge amount of material will need to be removed to bring it down to size. As long as you have a lathe with power feed this is not an issue other than the time expended watching the lathe remove material.
The first thing you need to do is to square up the ends of the blank and drill the bore for riding on a live center. A center drill or countersink works well for forming a live center bearing point. Due to the 24-inch length of the blank and the capabilities of your equipment, you may not be able to turn the complete 24 inches of blank at one time. There may be too much tool deflection or tool chatter when the cutter is far from the supported ends. On my lathe I could work no more than 18 inches at a time to keep deflection and chatter at acceptable levels.
Since barrel blanks may not have bores concentric to the outside diameters, you need to create a concentric outside diameter on the end that will be in the chuck. Since the bore is turning on the live center of the tail stock, simply true up a couple inches of the end to create a concentric section of barrel. This end will then be placed in the chuck. The other end will then be riding on the live center and it will true up as you are turning the barrel to size. If using a three-jaw chuck, verify that it is running true. Many three-jaw chucks are off a few thousandths and you may need to shim the jaws slightly to get the barrel running true.
The first thing I did after getting the blank set up was to turn the 1.375" blank down to a preliminary diameter. The widest diameter I would need was .966", which is where the rear sight base fits. I turned the blank to that size along the complete length. This equates to removing almost half the barrel blank, which took almost two hours of running time. With the largest dimeter turned, I focus my attention to turning the remaining diameters to the lengths required.
On my barrel this meant that I had nine specific diameters of varying lengths. These diameters (within a couple thousandths) were as follows: barrel shank that fits in trunion--.908", rear sight base diameter .966" (already done), section between sight base and handguard retainer .955", hand guard retainer area .944", fin area .920", gas block area .808", gas block to bipod pivot area .763" and bipod pivot area .592". The last diameter was for the threaded muzzle. This diameter will depend on what type of muzzle attachment you will be using. Standard AK47 muzzle threads are 14mm x 1 left hand threads. Since I did not have a 14x1 LH die or want to take the time to convert my lathe over to metric, I went with a more conventional size. I turned the threaded muzzle section to .560". This would leave enough material for cutting 9/16" x 24 threads (this size is also known as 9/16" extra-fine threads). I had this size tap so I could modify a regular YHM Phantom AR-15 muzzle attachment to this size. The AR-15 flash suppressor was drilled out to allow passage of a .30-caliber bullet and then drilled and tapped to 9/16 "x 24 tpi. This method was much easier than converting the lathe to metric and then trying to form left-hand threads.
Again, I must remind readers that all these listed sizes are approximate, you need to tune them to your specific needs. The trunion, sight bases, and gas block are interference or press fits. The diameters on the barrel need to be .0005" to .0015" OVER the hole size in the parts. This allows the parts to stretch and tightly grip the barrel when they are pressed in place. Some original AK47 parts may feature press fits of 3-4 thousandths under barrel diameters. This is only acceptable for a factory setting where they have all the tools and fixtures for supporting the parts solidly during this very tight fitting. Press fits this tight are very difficult for the hobbyist to do without damaging the parts. This is why I recommend a lighter .0005" to .0015" press fit. The two most critical press fits are the trunion fit and the gas block fit. The trunion fit must be tight enough to securely grip and support the barrel yet not be so tight that you risk cracking the trunion while pressing it in place. I sized my barrel for a .0015" press fit into the trunion. This is not as tight as the factory fit, but it is adequate and much easier. The gas block also needs to be similarly fitted. This will prevent any gas leakage that could blow gases back towards the operator. AKs are typically over-gassed so a little leaking won't effect functioning but leakage will effect safety.
While barrel diameters have to be very close to the specified sizes, you have a little more leeway when it comes to length. Some length sizes listed here are from the Green Mountain barrel print while others are based on my old parts or the new Green Mountain RPK barrel. Again, I caution readers to just take them as rough guides. Use your own judgment to determine your specific needs. Many length dimensions are not critical and the builder can vary them considerably without consequence.
The first length is the portion that presses into the trunion. This length was 1.480" to 1.500". The breech end has a large taper to ease insertion into the trunion and also to allow the barrel end to pass over the bullet guide in the trunion. The next length is for the rear sight base, and begins at the edge of the trunion area and goes forward 1.6". This can be slightly longer without concerns. The space between the sight base portion and the hand guard retainer area can be whatever remains after forming those two areas. According to the Green Mountain print, the centerline for the groove of the handguard retainer rotating half pin is located 7.392" from the breech. You need enough length on each side of this dimension to support the handguard retainer. Exact length is not critical. Now you need some space for the cooling fins. I made my fin space run from the edge of the handguard retainer area to a point 10.625" from the breech. The fin portion is not critical but the section just ahead of the fins must be located so the gas block can fit in the proper place. For the gas block section, I started at the 10.625 fine and went 1.5" forward. This will leave plenty of room for gas block mounting. Before determining the remaining lengths, I cut off the excess barrel length from the turned blank so I had an overall length of 21.33". I then worked backward from the muzzle. The portion for muzzle attachment threads was .700" long. From that .700" line I went rearward 2.125" to create the section where the front sight base and the bipod will be fitted. The length between the gas block area and the bipod area was whatever remained since that length is not important.
This is a lot of dimensions to remember! To keep all these figures in an orderly fashion I took the incomplete Green Mountain barrel print and added all this information so I would have a complete barrel diagram. Before you turn any dimension, you should double check to verify that the proposed dimensions will create what you actually need. It is easy to misread a print or write the wrong number in the wrong place and then blindly follow the print. You think your part will be to print but it will be wrong! Considering how much time and money is invested in this part, you want to be sure you get it right. Once you take off, steel it can't be put back on. Double or even triple check everything before you cut any steel. Also keep in mind that I may have made a mistake! I have also seen typographical errors in my articles when they were converted from my raw article format into the text you see here in Firearms News. Check! Check! Check!
The time to turn this barrel will vary depending on your equipment, skill level and other factors. I had 5 or 6 hours of labor into turning this blank to size. One last thing I will mention concerns forming the cooling fins. I was not concerned about recreating an exact duplicate of original fins. I just wanted a reasonable representation. To cut the fins I just put a 1/8" parting tool in the tool post and used it to cut 1/8" grooves 1/8" to 5/32" deep into the barrel. The grooves were placed 1/8" apart and featured slight 45 degree bevels on each side of the bottom of the groove so I would not have sharp corners in the grooves. The top of the fins were also lightly beveled with a file so the edges would not be sharp.
With the exterior formed on the barrel, it was time to cut a 7.62x39mm chamber. You will need a 7.62x39mm "GO" headspace gage and a 7.62x39mm finish chambering reamer. Reamers are available in finish or roughing styles. Roughing reamers do what the name implies; it cuts a rough chamber not quite to finished dimensions. This saves wear and tear on the finish reamer. For just a few jobs, you can get by with just a finish reamer. Doing the job with a finish reamer will be slower but it does save having to spend an extra $100 plus for a rougher. Your best source for headspace gages and chambering reamers is Brownells (www.brownells.com). If you do not want to purchase a reamer, they can be rented from online reamer rental vendors for about $30 per use. The chamber will be cut by turning the reamer into the barrel until the desired chamber depth is obtained. The depth dimension I chose was based on my purchased GM RPK barrel plus the dimensions from several AK-47s in my personal collection. The easiest way of precisely measuring the depth of the partially cut chamber is to measure how much of a headspace gage extends out of the rear of the barrel. I decided that my chamber would be at the right depth when my "GO" headspace gage extended out of the breech . 110"-. 112". The best tool for measuring the extension is a depth micrometer although a dial caliper will work if you are sure to hold it squarely on the gage.
A chambering reamer is a precision cutting tool. You cannot simply cram it into a barrel under power like a drill bit. Using a high spindle speed and a heavy feed rate will likely result in a poor quality chamber, and you risk breaking the reamer when the flutes fill with chips. Unlike a drill bit that cuts on the end, a chambering reamer cuts along its entire length. A reamer also does not have spiral flutes that that allow shavings to be driven up and out of the hole. When using a chambering reamer you will advance it slowly into the work at low rotational speed. You will only advance 1/16"-1/8" at a time before pulling the reamer out of the hole to remove shavings that have filled the flutes. You also will be using copious quantities of oil or cutting fluid in the hole and on the reamer.
Every gunsmith has their own preferred method of cutting a chamber. I will explain how I do it, but my way certainly is not the only or even the best method, it's just the way I do it. Before beginning the cut, I take a cartridge and lay it beside my reamer so that it closely follows the shape of the reamer. I then mark the reamer at a point that represents about 1/4" less than full chamber depth. I mark the reamer with tape and a black marker line. This gives me a positive stopping point to prevent over-cutting the chamber. I place my barrel in my lathe for preliminary chamber cutting making sure it is set up true. I cut about 75% of the chamber under lathe power just to save myself a lot of hand turning. A chamber can, however, be cut totally by hand if you chose. I set my lathe speed at its lowest speed which is 90RPM on my Chinese lathe. While I do use the lathe to spin the barrel I do not feed the reamer into the barrel with the tail stock. I feed it in totally under hand pressure. I place my reamer into a tap handle and use a spring loaded center in the tail stock to keep the reamer square to the work. Since I am only using a finish reamer, it will be removing a lot of material. It will be advanced into the barrel slowly with a lot of stops to remove shavings and add lubricant. I begin by placing the reamer pilot into the bore and applying a lot of oil to the bore and reamer. With the lathe running slow I slowly feed the reamer into the barrel. I hold the tap handle loosely in case the reamer would bind from chip build up or too heavy of feed. The reason for a loose hold is so that if the reamer binds it will slip out of my hand and begin spinning with the barrel rather than breaking. Think of the loose hold as manual safety clutch. At the beginning you will only be cutting the chamber lead so you are not taking off much material so you can go in about 1/4" before pulling out for chip removal. Once you start removing a lot of material you should pull the reamer out for shaving removal and lubrication every 1/16" to 1/8". I lube each time I start a cut and also once or twice as I cut. Clean shavings out of both the reamer flutes and the chamber. You can use a brush for this, but I like to use an air gun to blow out the shavings and prevent them from building up in the barrel's bore. Doing a dozen or more cut, clean and lube procedures is not uncommon during a chamber cutting.
I cut in the lathe until I have only about 1/4" more to go and then remove the barrel from the lathe. I then set the barrel up vertically in a bench vise for final cutting. The last 1/4" will be cut totally by hand. The first thing I do is to clean out all shavings and then place the headspace gage into the partially cut chamber and take a preliminary measurement. Be sure that both the chamber and the gage are completely clean each time you measure. One little metal shaving or piece of dirt can throw off your dimensions. My first measurement indicated that I still had about .200" to meet my desired dimension of .111" of gage extension. Before cutting more, I remarked my reamer to indicate another .150" of cutting depth to prevent cutting too deep. You want to have a definite stopping point that can't be missed to prevent over-cutting. I then continued cutting till I only had .050" to .060" to go. Once you get this close to final depth you need to cut and re-measure many times to prevent cutting too deep. If you over cut due to inattention you may ruin your barrel. When you get to within a few thousandths of final depth, you need to cut with little pressure on the reamer. It takes very little pressure or turning to remove only a couple thousandths of an inch. Once I had the depth cut so that the gage extended out. 111" my chamber was done except for a final polishing later.
If, by some chance, you mess up and cut too deep all may not be lost. If you are only a few thousandths (.010".-015") too deep this may be able to be compensated for during headspace setting later. If you really screwed up and cut way too deep (like a 1/8" too deep), your barrel is likely ruined for this project.
This RPK barrel, like most other AK-47 barrels, features a light chamfering of the chamber. A 30-degree chamfer needs formed to ease cartridge chambering. This 30 degree chamfer is .640" in diameter and is .035"-.040" deep. If you are measuring the width of this chamfer it is about .090". I cut this chamfer on my lathe but it could also be cut with a countersink (with correct angles on it) in a hand drill. Exact size is not critical as long as you don't get to far from this size.
Now a recess needs formed on the breech for extractor clearance. This recess runs from about the 11 o'clock mark to about the 4 o'clock point. The easiest way of making this recess is to use a 1/4" diameter end mill that has about 1" of usable cutting edge. Exact recess size is not critical as long as you have adequate clearance for the extractor. To indicate the area to be recessed, I marked the breech with layout fluid and then scribed lines to indicate the area to be cut away. The recess is basically just a 90 degree cut that is .035" to .040" deep. The line to represent the upper edge was .550" from the right side while the bottom line was .285" up from the bottom. I ran my small mill up to the upper edge till the radius was touching the line. The bottom was square and even with the bottom line.
Once the recess was cut there was one last thing that needed done to complete the chamber work. I lightly chamfered or "broke" all sharp edges with a small fine-cut round file. The filed edges were then smoothed up with 320 grit sand paper. This was followed by polishing with a felt bob coated with jewelers rogue in a Dremel tool. I also polished the interior of the chamber with my Dremel tool. Once this was done the barrel work was completed except for installation in the trunion and drilling the gas port later.
It was now time to turn my attention to receiver fabrication for the economy project. To make a receiver from the AK-Builder frame flat, you will need a flat-bending fixture. These can be purchased from AK-Builder and several other sources. Prices can run from a low of about $100 up to about $250. There is, however, a way of saving the cost of a purchased bending fixture. In the Shotgun News Treasury #14 (available through the Firearms News website) there is an article titled "AK-HD, Building a Heavy Duty Semi-Auto AK-47 Receiver". This article shows readers how to make an AK frame flat bending fixture for less than $25. The article goes on to show how to use the fixture to make a 1.5mm heavyweight AK receiver. To use the featured fixture to make the RPK receiver the dimensions will need to be modified slightly. A standard Euro format AK receiver features trunions that are approximately 1.250" wide. The Yugo RPK receiver features trunions that are 1.200" wide below the trunion bulges. The width of the punch and die will need to be modified to work on the narrower RPK receiver. I made the small modifications needed to the fixture and it worked fine. In fact I modified my dimensions so that I could still use the fixture for any size of receiver. Because the fixture article is a very long two part article I do not have the space to cover the receiver fixture fabrication or how to use it in this article. I highly recommend that you purchase the Treasury for the fixture article plus all the other articles written by many other Firearms News writers. Because the Treasury article goes into considerable depth, I will only cover some of the major points of receiver fabrication in this article.
One thing I should mention about the article in the Treasury is that the bend fixture was for forming my own design of frame flat. My self-made flats did not feature the protruding magazine stabilizers found on normal AK receiver flats. If you use normal frame flats, you will need to form stabilizer reliefs in the punch so they can bend completely. I added these reliefs after I wrote that article when I needed them on another flat bending job that featured a flat with the magazine stabilizers.
An AK receiver flat for this and other projects is simply formed by forming a flat around a punch by forcing it into a die. The punch is sized to the internal dimension of the receiver (1.2" for the RPK receiver) and the die is sized to the exterior dimension plus a few thousandths for clearance. To accurately bend the flat, it must be precisely centered on the punch and securely clamped in place to prevent movement during forming. The flat must also be clamped in place to prevent it from pulling away from the punch while several tons of force is applied with a hydraulic press. Besides a thick clamping plate, my fixture features locating pins to further align and hold the flat in place during bending.
AK-Builder frame flats are considered to be the best flat on the market. All you need to do is to bend it to shape and then install the trunions from the kit and the lower rails that are supplied with the AK-Builder flat.
The Yugoslavian RPK receiver flat is made to fit what is known as "bulged trunions". This means that the front trunion is not flat on the side like a common Euro-format AK receiver trunion. The Yugo trunions feature bulges that extent above the lower edges about 1/8". Since my fixture is made for normal flat trunnions, the Yugo flat cannot be pressed all the way into the die during forming. This is not an issue since the bend is already made before the trunion bulges contact the fixture.
The first operation that needs done is that the flat must be clamped to the punch. It must be precisely centered on the punch for proper forming. If not centered, you will end up with one side that is taller than the other. If you somehow end up with one side a little higher than the other don't panic. As long as both sides are within .010" to .015" of each other the receiver should still be usable. The AK-Builder flat does not feature holes for locating pins, so after I got my flat precisely centered on the punch, I drilled through my existing pinholes and formed holes for my pins and then installed them. You don't have to have locating pins, but it does offer some extra support for maintaining exact alignment of the flat on the punch.
Once the flat was precisely located on the punch, it was set over the die in the bend fixture. To bend correctly, the punch needs to be centered over the die. This was achieved by installing two U-shaped pieces of scrap steel the same size as the receiver wall over the punch and down into the die. This will keep the punch centered as it is pressed into the die. Due to the great force required to force the punch into the die, you should use an additional piece of heavy steel over the punch. I used a piece of 1'x1.5" flat stock to distribute the force over the complete punch rather than just exerting force where the ram presses against the punch. Failure to distribute the press tonnage evenly over the punch can result in bending the punch on heavy flats such as this 1.5mm (.062") flat. Prior to bending the flat, you should apply some heavy grease to the die and frame flat where they will contact each other during bending. This will ease bending and also help prevent scratching on the flat as it is being forced into the die.
Once I had the flat set up over the die, it was time to begin bending. The tonnage required to bend the flat will vary depending on the type of fixture used, flat thickness and the alloy of the steel. I have made AK receivers from .078" thick 4130 steel and it took almost the full 20-ton capacity of my press to start the bend. Standard receiver flats (1mm or .040" thick) may require only 6 to 10 tons to bend. I estimate that the AK-Builder 1.5mm RPK flat took about 8-12 tons of force to start the bend in my self-made fixture. Maximum force is required to start the bend. Once the bend begins the force required to continue bending drops off dramatically. I pressed the flat and punch into the die until the trunion bulges just touched the top of my fixture. Because the flat is tightly pressed into the die, it will need to be forcefully removed from the die. On my fixture, I have large holes in the bottom to allow me to tap the punch out with a large piece of brass rod. Once the flat is removed from the die, you will likely have a bent flat that is not quite a full 90 degrees. This is due to spring back from that strong steel flat. To bring my sides up to almost a full 90 degrees, I place the still assembled flat and punch in a large vice and squeeze the bottom tightly. This will most times bring the sides to a full 90 degrees. Sometimes, if using extremely stiff steel, the sides may still not be all the way to 90 degrees. In these cases I just take the receiver off the punch and then place the bent receiver in a vise and squeeze it to finish the bend.
After you remove the bent flat from the punch you should check to verify that the flat bent correctly. When I checked, both the flats I bent the sides were within acceptable height tolerances. There is one issue common to bending these very thick and strong 1.5mm flats: At the magazine well area, a very short flange is being bent. Due to the strength of the material and the fact that there is little material to hold on to, the edge of the magazine well may not form well. Sometimes the very short edge will pull away from the punch rather than bending. If this happens all you have to do is hammer the steel over the punch to finish the bend. If you hammer it carefully it should look reasonably good. This problem typically does not occur with thinner receiver flats and I have had this issue occur only with very thick and strong flats.
Some receiver flats come with the trunion rivet holes already drilled. Due to the wide tolerances found on AKs, many times drilling the holes yourself results in a better fit. On my AK-Builder flat the rear trunion holes were already accurately located and drilled, while the front holes were absent. Before I could drill the front trunion rivet holes, I had to fit the trunion to the formed receiver. The front bulged trunion found on Yugo AKs feature very sharp edges on the raised bulges. Due to the difficulty in forming sheet steel with this tight of bends the receiver flats typically have rather broad radiuses on the trunion bulges. To get the trunion to fit up tight against the sheet steel receiver, you will have to fine tune the fit of the parts so that there is no gap between the parts. The parts need to be fitted close together so that when riveted together the parts are nice and tight.
You have a few choices on how to install the rivets. Rivets can be set with rivet forming tools that look like bolt cutters, they can be set with the aid of a hydraulic press and riveting fixtures, or they can be set with nothing more than a hammer and some self-made rivet head forming punches. I have used all of these methods and many times use multiple methods on the same project since some processes work better than others in specific cases. Over the years that I have made over a dozen AKs I have pretty much went with the hammer and self-made punch method due to the low cost and the fact that hand forming rivets is extremely easy. On this project I decided to try using the "bolt cutter" type of rivet setting tool just to try a new method. Rather than buying one of these tools I made one myself for less than $25 by utilizing a set of 18-inch bolt cutters that were obtained from my local Harbor Freight tool store.
The rivets that came in my set of AK-Builder rivets featured heads larger than those of the 1/8" rivets I would use on my lower rails. Once the threads are removed from set screws, there would not be enough material around the dimple to fully support the larger rivet head during forming. To gain some extra material thickness, I used 3/8" by 3/4" long grade 5 hex bolts. I turned off the hex head so that I had about 1/4" of full diameter remaining at the front of the punch material. By having a full 3/8" I would have enough support to form a larger head. I used the same method to create representations of rivet heads in the larger punches. Because these bolts did not have Allen wrench sockets in the end I just cut a slot in the bottom with a hack saw. This would allow adjusting with a small screw driver.
These two punch sets would take care of many of the rivets I would need to set. The rivets that could not be accessed or set with the new tool would be set the old fashioned way with simple rivet punches and some heavy back up blocks.
In the next part of this article series I will begin assembling the receiver. This will entail installing the front and rear trunions, lower rails, and trigger guard. It will also cover fitting the bolt carrier to the receiver and several other assembly procedures.
Chambering Reamers and Headspace Gages // Brownells, 200 S. Front St., Montezuma, IA, 50171, 800-741-0015, www.brownells.com
7.62x39mm Barrel Blank // Green Mountain Rifle Barrel Co., R O. Box 2670, Conway, NH 03818, 603-447-1095, www.gmrlflebarrel.com
RPK (M72) AK-47 Receiver Flat // AK-Builder, 3625 SW US 40 Hwy., Blue Springs, MO 64015, 816-229-0804, www.ak-bullder.com