Friday, March 30, 2012

Building a semi auto RPD rifle

This post is about my efforts to work on another project as I digress temporarily from working on the Suomi drum/Hi Point match up. Follow along and see what it takes to build what is essentially a belt-fed AK47!

One of the projects I picked up recently was a Polish RPD parts kit. The good folks at MGS sold me a complete parts kit, minus receiver, and every part was in great shape. I do not think this rifle was ever even fired before being demilled. Instead of being torch cut, it appears that the receiver was cut with a chop saw originally. The front receiver stub that was still attached to the barrel was still usable, and I took advantage of this.
I called Sarco and ordered a demilled receiver and about three weeks later, I got in a big pile of slag-encrusted receiver parts. Apparently, whoever demilled the receiver did so with a torch, and it looks like they had beaten the parts apart with a hammer while they were still hot. I was able to get them straightened out somewhat, but they are far from parallel perfect.

Now, since I was using a demilled receiver, the ATF mandates that the receiver cannot be reassembled in a manner that would allow a full auto bolt or bolt carrier to fit or function in it. To prevent any legal issues on this front, I welded up one inch of the rails under the receiver nose so that a standard bolt carrier would no longer fit. In modifying the bolt and bolt carrier, I milled off one inch of the rails on the front of the bolt carrier. I then milled a slot in the bolt itself, and welded a denial pin in the side of the receiver nose stub. This pin would prevent a full auto bolt from fully closing and allowing a round to be fired. I also milled off the stub that impacts the firing pin as the bolt closes, and I also ground off the ridge that holds the bolt carrier to the full auto sear.  I will not be using the original trigger or sear, and plan on modifying the rifle to accept a AR15 hammer/disconnector. This will be a huge difference over the original open bolt system, and will allow for a much better trigger than is available from the semi auto striker systems that are currently available for this rifle. This also solves part of the 922R issues that arise when building a semi auto rifle out of a imported parts kit.

Here is a short quote of the law, and a list of parts that need to be thought about when building one of these rifles.
(a) No person shall assemble a semiautomatic rifle or any shotgun using more than 10 of the imported parts listed in paragraph (c) of this section if the assembled firearm is prohibited from importation under section 925(d)(3) as not being particularly suitable for or readily adaptable to sporting purposes.

Paragraph (C) defines the following parts as "countable" under the law:
(1) Frames, receivers, receiver castings, forgings or stampings *
(2) Barrels
(3) Barrel extensions
(4) Mounting blocks (trunnions)
(5) Muzzle attachments
(6) Bolts
(7) Bolt carriers
(8) Operating rods
(9) Gas pistons
(10) Trigger housings
(11) Triggers
(12) Hammers
(13) Sears
(14) Disconnectors
(15) Butt stocks
(16) Pistol grips 
(17) Forearms, hand guards
(18) Magazine bodies
(19) Followers
(20) Floor plates

Numbers 3, 18, 19, 20 are non-issues. This rifle is belt fed, so there are no magazines and there is no barrel extension.

I will be either using American parts for the following, or making my own:

1. Muzzle brake, so number 5 is out.
2. Gas piston, so number 9 is out
3. Trigger housing, so number 10 is out.
4. Trigger, so number 11 is out
5. Hammer, so number 12 is out
6. Disconnector, so number 14 is out.
7. Pistol grip, so number 16 is out.

This leaves me with 9 imported parts, which puts me in compliance with the law. I may reuse the original gas piston, which puts me at 10 imported parts, still not a legal problem. As such, the decision to use AR15 fire control parts made the list easier to deal with. It's still a silly law that has nothing to do with reducing crime, and only serves to make it harder for us Americans to import or build otherwise perfectly safe and usable weapons.

There will be another post entirely about the modifications to the rifle and receiver in order to make it function with the original recoil spring assembly and a hammer fire control system, as the recoil rod and spring are right in the middle of where the hammer needs to go. I haven't had time to work on this yet, but already know how it will be done.

In this post, I will outline the work done on the receiver so far, including the welding jig I had to machine to line up all the parts and get them welded with out burning holes in the receiver.

To start, I took some important measurements from the receiver side plates I had, and ordered some copper plate from Speedy Metals. I transferred the negative image of the receiver side plates to the copper plate on the milling machine.
Here's the jig being milled for use.

 This is what the receiver side plates look like after being cleaned up and de-slagged. They aren't complex and should be pretty easy to copy if receivers continue to be hard to find.

Here the top and bottom slots have been milled out, now it's time to mill out the middle slot.

 One thing I must mention is that the slots inside the receiver pieces have slight radius's in the corners, so the square-cut welding jig wouldn't fit quite right. I had to put the receiver pieces in the mill and very carefully mill out the corner radius's so the jig fit very tightly.

Now, for the fun part. The receiver has had denial pins welded in, the lower carrier slots were welded in as a denial method for the carrier, and the receiver can now be made one piece. I took some careful measurements and milled the original receiver nose stub down square, and then took the nose stub off the torch-demilled receiver from Sarco and milled it to length and square. I beveled the edges for welding, then started clamping.

You can't tell from this pic, but I had to modify the welding jig slightly to fit around the denial areas of the receiver nose.

Here is the welded 2/3 receiver with the belt feed mechanism in place. Notice that the last 1/3 of the receiver is not welded together just yet, since several modifications need to be made in order to keep the receiver in full compliance with the law, as well as allow for an AR15 FCG to be used.

Here is part of the welded receiver after being sanded down. You can see the discoloration from welding the inside of the receiver. So far, so good!

Here is evidence of my first mistake in this build. I forgot about the non-reciprocating bolt handle having to travel in this slot in the side. By the time I got to this point, I was very hot and tired having been milling, grinding, and welding on this and other projects. So here I am, welding, and very happy with the progress when I notice I've just welded through my bolt handle slot area. D'OH!
I've since cleaned this up some with a Dremel tool, but it sure was frustrating at the time.



  1. Looks interesting. You have to start your own firearms-business, that's for sure! :)

    1. Thank you Hyperprapor!

      I could start a business but then it wouldn't be fun =)

      Wait until you see all the AR 15 receivers I have here to make.

    2. Name's Alexander. Hyperprapor - just a nickname, for blogs. :)
      Well, that should be something. Will wait.

    3. Glad to finally know your name, Much easier to spell.

      There's a new post up just for you, lots of work to be done there.

  2. I am thinking of building my first AR15 and don't know where to look for the best quality parts. Can someone recommend some good ar15 lower receiver manufacturers?

    matched receiver sets

    1. A lot of the receivers on the market today are all made in the same place. There's just not a lot of difference between manufacturers when it comes to milspec lowers.
      Uppers are a whole different can of worms, due to options available.
      I've always had good luck with Palmetto state armory, centerfiresystems, and model one sales.
      Check over at and ask the guys there. You'll get lots of opinions, but lots of options.