By Frank Iannamico
Brief History of the RPD
The Soviet Ruchnoy Pulemet Degtyaryeva (RPD) light machine gun was developed near the end of World War II, shortly after Russians adopted the destined to be famous M43 7.62x39mm cartridge. However, the RPD was not issued in large numbers until the 1950s. Although some considered the intermediate 7.62x39mm round too anemic for use in a light machine gun, testing revealed that lethality of the 123 grain bullets was quite satisfactory at normal combat engagement distances. The smaller cartridge allowed the infantryman to field a truly “light” belt-fed machine gun. The use of a fixed barrel kept the weight to a minimum and the design uncomplicated. The operator of the RPD was trained to fire the weapon in short bursts to extend barrel life.
Like most Soviet weapons, the RPD was named after its designer- Vasiliy Degtyarev- with the assistance of his design bureau staff. During the Cold War period, the RPD saw widespread use when it became the standard squad automatic weapon (SAW) for the Soviet Union, the armies of the Warsaw Pact countries, and Soviet satellite nations.
The RPD light machine gun was relatively unknown to most in the West until the United States entered what would become known as the Vietnam War. During that conflict U.S. infantrymen became well aware of the RPD that was used effectively against them by the North Vietnamese Army (NVA) and the Viet Cong guerrillas (VC). The RPD, with its cyclic rate of 650 to 750 rounds per minute, was especially effective when used by the enemy to ambush U.S., Allied and ARVN troops.
The 7.62x39mm RPD machine gun is a gas operated, locked-breech, fully automatic only weapon that fires from an open bolt. The weapon is fed from two non-disintegrating, 50-round belts that are linked together by the insertion of a cartridge. The belts are normally contained inside a steel drum, which attaches to a bracket on the weapon. To prepare the RPD for firing, the bolt handle is pulled to the rear and a belt is fed into the feed tray by a starter tab, which is attached to one of the belts. Once the trigger is pulled, the bolt group moves forward through the energy of the compressed recoil spring located inside the buttstock. Upon its forward motion the bolt strips a cartridge from the feed belt and chambers it in the barrel. The breech is locked when its two “lugs” are engaged by a wedge on the bolt carrier forcing them outward into recesses machined into the sides of the receiver. Once the breech is locked the wedge continues forward until it strikes the firing pin, igniting the cartridge’s primer. Some of the gas pressure generated by the fired round is forced down through a port in the barrel, which acts on the head of a gas piston forcing it and the bolt carrier rearward, retracting the locking lugs. The belt is fed by a pawl that is actuated by a stud on the bolt carrier that fits into a curved track on the top cover.
The Semiautomatic RPD
A transferable full-automatic RPD machine gun is quite rare, and when found can be prohibitively expensive, generally somewhere in the mid five-figure area. With the recent importation of like-new RPD parts sets (less receiver), most of Polish origin, it was only a matter of time before someone figured out a way to make a semiautomatic version that would pass the scrutiny of the BATF’s Technical Branch. To be legally acceptable, one of the most important requirements was that the weapon fire from a closed bolt and secondly not be easily converted to a full-automatic operation. This presented two problems; designing a semiautomatic-only receiver and a trigger group.
Vector arms once offered a semiautomatic RPD, but they were quite expensive. More recently DSA Arms has manufactured excellent quality receivers and complete semiautomatic RPD “rifles” using many original RPD parts, as well as their RPD Carbine, a modernized machine gun.
This article offers some basic instructions on how to build a semiautomatic RPD for those who like such DIY projects. However, the project is quite involved and requires substantial skill and machine tools to complete the job. Many of the instructions for a semiautomatic RPD build found on the internet are incomplete and/or are not dimensionally accurate. The machining and fitting of many individual parts and adjusting the headspace are best left to an experienced gunsmith with a well equipped shop. For the build in this article, Tony Veronesi of Veronesi Gunsmithing was enlisted for the project.
Building a semiautomatic RPD must be done according to BATF approved methods, one of which is the firearm must fire from a closed bolt. The build must also use a specified number of U.S. made parts. The build described in this article has a U.S. made barrel, semi-auto receiver and a U.S. made fire control group. In addition, many of the original RPD parts were machined to function in a semiautomatic-only configuration. The semiautomatic fire control parts are available from several sources and include instructions on location of holes and machining dimensions. The instructions were not published here due to copyright considerations.
If your RPD parts kit came with an original barrel, the build will be somewhat easier because it will eliminate the fitting of a few parts. Unfortunately, since kits can no longer be imported with barrels, most of the kits have U.S. made stripped barrels. However, if an original barrel is used, the builder must insure there are enough U.S. made parts used to be 922 (r) compliant.
Bolt Carrier
The original bolt carrier needs several machining operations in order to convert it to function in a semiautomatic configuration. The semiautomatic RPD receiver, as designed, has bolt carrier grooves in the receiver that are narrower than an original so that it will not accept an unmodified bolt carrier. To get the bolt carrier to fit, the top of the side rails of the carrier needs to be machined to fit in the receiver. To determine how much metal to remove is best done by first measuring the receiver grooves, allowing for enough clearance to ensure smooth operation.
The sear surface on the bottom of the carrier needs to be milled off, leaving the sides of the carrier intact. The front portion of the bolt carrier wedge that originally contacted the firing pin needs be machined away enough that it will no longer contact the firing pin. In the same area of the bolt carrier a hole needs to be drilled to accommodate the firing pin extension, which comes as part of the semiautomatic fire control group. The hole must be carefully located so that the extension will be directly in line with the firing pin in the bolt. The firing pin extension is then secured in place by drilling a hole and installing a crosspin.
Barrel
Some RPD barrels are threaded into the receiver, while others were pressed and pinned into place. The DSA semiautomatic receiver for this project was designed for a press fit. The barrel for this project was a new U.S. manufactured part with threads, but there was no extractor notch, requiring several machining operations. One was to remove the barrel threads, and machine the chamber end of the barrel for a tight interference fit in the receiver. The second operation was to locate and machine a notch for the extractor.
There are several components that need to be fitted onto the barrel: the gas tube, gas block and front sight tower. Generally, new barrels are slightly oversize in the locations these components are located. To ensure a tight fit, these areas will need to be machined to size on a lathe. After sizing and fitting, the components should not be pinned in place until the barrel is pressed into the receiver.
The barrel is a tight interference fit pressed into the receiver with a hydraulic press. During this operation it is important to keep the barrel straight and the extractor notch in the correct orientation. The barrel is pressed until it butts against a raised surface inside the receiver. Before proceeding further, the bolt and bolt carrier need to be installed in the receiver with its two locking lugs in place. With the bolt fully forward the lugs should not lock in place at this point indicating a tight headspace. The headspace will be adjusted later by careful grinding of the back of the lugs to fit and lock in the receiver. After determining the headspace is tight, the crosspin to secure the barrel in place can be installed. The DSA receiver has a dimple at the correct location for the pin. The pin must be a tight fit to keep it from working out.
Adjusting Headspace
The bolt of the RPD is locked in battery by the two lugs located on both sides of the bolt. When the bolt is fully forward the lugs are spread apart by a wedge shaped protrusion on the bolt carrier and locked into recesses machined on the inside of the receiver. Headspace is adjusted by the precision grinding of the rear end surface of each locking lug. To adjust the headspace of the build, a set of 7.62x39mm GO, NO-GO headspace gages and a set of feeler gages are needed. A precision surface grinder is needed to remove material from the rear of the lugs. As per earlier instructions, at this point the lugs should be too long to lock in the receiver when the bolt is closed. Material needs to be carefully ground from each locking lug individually until there is .005-inch clearance between the locking lugs and the receiver. After adjustment, the headspace should be checked by installing the GO gage, closing the bolt and insuring the lugs lock into the receiver with the gage in place. Next install the NO GO gage. The lugs should not lock in place with the bolt forward.
After the barrel is installed the barrel components can be permanently attached. The gas tube can be located by using the gas piston as a guide by carefully centering the piston in the tube. After all of the parts are correctly aligned they can be drilled and pinned in place. There should be a 13mm gap between the front of the gas tube and rear of the gas block. After the gas block is permanently in place the existing gas port in the block can be used as a guide to drill a corresponding hole in the barrel. After the gas block and tube are installed the front sight can be aligned and pinned in place.
The drum bracket was originally riveted to the side of the receiver. The DSA receiver has three threaded holes to secure the bracket with countersunk allen-head screws. Loctite is recommended to keep the screws from loosening. The top cover and feed tray may need fitting to the receiver; they are secured by a long rivet.
Trigger Frame
In order to fit the new semiautomatic fire control group in the original machine gun trigger frame, several machining operations need to be preformed and several holes drilled.
By design, the machine gun trigger frame will not fit on the semiautomatic receiver because the trigger frame is too long. The front end must be trimmed so that the back of the frame butts snug against the receiver and allows the take-down pin to be installed.
The next three axis pin holes need to be correctly located and drilled in the trigger frame. Drawings that come with the semiautomatic fire control parts indicate the location of the holes. The locations of the holes are on a curved surface making them difficult to drill accurately. This problem can be solved by milling a small flat surface at the location of the hole to be drilled. The semiautomatic parts kit included roll pins for the fire control axis pins. For this project solid axis pins were fabricated from the shanks of drill bits.
Trigger
The original machine gun trigger is used for the conversion, but it must be machined to accept the return spring included in the kit.
Safety
The original safety lever is modified to block the sear movement to keep the weapon from firing when the lever is in the safe position. This is done by welding additional metal to the original safety and machining it to size.
Resources:
Veronesi Gunsmithing
Seminole, PA 16253
Phone: (814) 275-4382
RPD Parts:
Receivers, parts and complete semiautomatic RPD rifles:
DSA Arms
Phone: (847) 277-7285
Web: www.dsarms.com/
Semiautomatic RPD parts and receivers:
Project Guns, LLC
Phone: (561) 394-8228
contact@projectguns.com
RPD part sets:
SARCO
Phone: (610) 250-3960
Web: www.sarcoinc.com/
Wise Lite Arms
Web: www.wlarms.com/
This article first appeared in SmallArmsReview.com on May 24, 2013 |
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