Thursday, July 12, 2012

Time for some RPD work....

I have been working on Suomis non-stop lately, and I've gotten kind of tired of the monotony of it. It's been nice to make some money on the side, but I'm kinda glad I haven't taken on any new orders as of late. Since I'm a bit worn out on building Suomis, and since today was my birthday, I decided to work on one of my own projects that I haven't touched in a while, the RPD rifle.
Here's a comparison of the
Original rifle
a DSA modified model.
Some background on the RPD........
The RPD was a Russian assault rifle, and was a decent belt fed machinegun in it's own right, IMHO. I have never shot one in full auto, but it was single-man portable, held 100-round belts of ammo that conveniently stored in a small drum, the belt links were non-disintegrating, and it shot a medium range rifle round. What's not to like?
Though the original design of the RPD has it looking quasi-RPK-ish with it's "club foot" buttstock, the bolt design, ammo feed design, and FCG are all it's own. The RPD fires originally from an open bolt, which means we have some serious reworking to do in our future.
As mentioned previously on this blog, since 1986 when the Hughes amendment (spit) was unlawfully passed, the construction of civilian transferable machine guns is outlawed. (The BATFE has ruled long ago that a open bolt FCG automatically construes a machine gun*)
The Treasury Department/BATFE will no longer accept a tax paid on the construction of new machine guns UNLESS they are made by an 07/SOT who is making them for military and/or law enforcement use.
I am not one of those groups, and I'm not an FFL of any kind, so it looks like I will have to build this rifle into a semi-auto.
To do that, I must modify all of the working parts of the rifle that allow it to fire from an open bolt, and ensure that not only will they fire in a semiautomatic manner, but that full auto parts can never function in the firearm. One thing I already did was weld in denial areas on the front of the bolt channel, so that a full auto bolt will never fit in the receiver, then after my bolt is modified to semi auto, I'll grind that much of the bolt rails off the carrier so that it will chamber a round.

Previously, I had began to work on the RPD, with a game plan of making the rifle hammer-fired, instead of the conventional striker-fired systems that are so prevalent with these guns in the aftermarket.
In my opinion, the striker system is less than optimal in these rifles, and I'll do what I have to do in order to prevent using one. I don't want a mushy trigger, long trigger pull, or anything else associated with striker FCG in this old rifle. What do I want? Well, for all intensive purposes, I want an AR trigger, or at the absolute least, an AK trigger.
Since this rifle is of Russian origin (though mine is a Polish-built example), it made sense to integrate a AK trigger group.
The AK trigger group would not come close to fitting in the place of the old trigger/sear spot, so I machined the bottom off of the lower receiver so I would have a flat area to work with when I get around to building the FCG housing. That is coming down the line, first I have to have the receiver built.

Now, there is a recoil spring that is held captive in the buttstock that has a rod attached to it. This rod protrudes through the front of the stock into the receiver, and fits into a divot on the bolt carrier. In normal operation, the bolt carrier is at the full rear position, the trigger is pulled, the sear drops and releases the bolt carrier, which slides forward, strips a round from the belt, and as soon as the round is chambered, the gun fires.
That recoil rod is going to be in the way of a hammer impacting a firing pin.
There are two ways to work around this.
1. Machine a new hammer that is clearance cut to miss the recoil rod and strike a large pad on the rear of the firing pin
or 2. machine a block that attaches to the bolt carrier and has the recoil rod pressing on it, and machine a hole in the middle of the block that allows a normal AK hammer to pass through.

I picked option 2. While it's pretty straightforward, it means that I have to lengthen the receiver just a bit to allow the correct amount of bolt travel with the new bolt extension in place. I figured one inch of extra length will work fine. Since I had already done some reconstruction and welding on the receiver, I picked up where I left off.

So, the first thing I had to do was take some measurements off the inside and outside of the receiver so I could machine some new pieces to fill in the gaps left behind from the drunken monkey that cut the receiver with a torch.

In this picture, you can see how large the gap is between the torched sections. This was about a half-inch gap. Not bad, and though it could have been filled with some scrap metal and some weld, I chose to mill out some new filler pieces.
 Here is a picture of the receiver as it sits, with some re-weld work done already, indicated by the visible heat marks present. (Not shown here is the area inside the receiver where the bolt channel was welded to prevent a full auto bolt from fitting in.)
I'm not a fan of trying to fit new receiver sections in with angled cuts on both ends, so....

A protractor gave me a straight reference point to scribe a straightedge onto.

Here is the new side plate I machined. Some may notice that the insides of the new filler pieces are machined to match the inside of the receiver as well.

After some creative cutting, grinding, welding, more grinding, etc., we now have one side done. Please excuse the mess on the workbench. I tend to spend more time building than I do cleaning, and it often shows in how messy my work area gets.

A closer view of the welded and sanded area. There are some low spots that I will fill in with weld before sanding smooth again.

Here you can see the one inch difference in the bottom channel of the receiver. After I trim about .5" off the length of the lower receiver, this whole remaining slot will be filled with weld so that a full auto lower receiver can not fit on the main receiver..

Here you can see the gap between the top cover and the rear of the receiver. This amounts to exactly one inch that I will have stretched the receiver.
I have yet to finish the other side, but it will be done in a similar manner, with the exception that a relief will be milled for the charging handle. Now, for those of you reading this who are attempting to build an RPD, and you either don't have a complete receiver, or you are building your own, here are the measurements of the side plates, except for overall length.
This is for reference only, or for machining your own reweld jig.

Use this at your own risk. I took these measurements with a Harbor Freight digital caliper and they all worked well on my reweld jig as well as the new filler pieces I machined. Everything lines up perfectly.
When I'm done with this build, I'll likely put up full dimensions of the receiver for those interested.

That's all for now, believe it or not that took about 5 hours to do, from machining the new pieces to welding and sanding.
Now on to the other side, and then back to the Suomis!

This has nothing to do with RPD's. This is the Chloe dog, resting on my bed. She is our initial line of home defense, as mentioned in my previous post. For those of you who have never met our canine friend, she is an absolute sweetheart,** and she is BIG. That is a King size bed she is sprawled across, and yes, her hind legs are almost at one end of the foot of the bed, while her front paws are at the opposite side of the foot of the bed. She is just this side of being a barking horse.

*The BATFE ruled an open bolt is a machine gun, except on guns that they say aren't machineguns.There is a big debate with the BATFE over a number of MAC10 style open bolt pistols that they declared were not machineguns, due to date of manufacture. More of the BATFE's shenanigans, nothing new here, move along.

**Unless you are not welcome in her house.


  1. any chance that bad boy will be up and running by DAB II?

    1. Kx59,

      Maybe. Don't know. =)
      You might have to show up at the blog shoot to find out for sure! =)

      FWIW, if it does put in an appearance, it won't look anything like it does now.

  2. Replies
    1. Thanks for stopping by and thanks for the nice comment, drjim.

    2. You're welcome.
      My Dad was a Tool and Die maker, and I appreciate fine craftsmanship.
      I *know* how much work it takes to build up something like that, even though I've never done any gunsmithing.

    3. As I've always said, I'm no expert, not a "real" gunsmith.
      Just a redneck playing with power tools.

      I've got an update to this build I need to post, just havent had time.