Semi RPD Build [Part 2] - Bolt, Barrel, Headspace

Welcome to Part 2.


In this post, we will cover bolt modifications, barrel prep and installation, as well as headspacing. The order of operations is as follows: barrel installation will be done first, followed by bolt modification and finally headspacing.

The Barrel.

We were very fortunate to obtain intact-barrel kits, because this build actually began as a cut-barrel build. Little did we know that demilling the old barrel stubs was so painstaking that it alone took up most the time that was dedicated to the build session (roughly week-long). But even if you get that far - there is still the matter of cutting the extractor relief.

Before doing anything else the the barrel, let's strip all removable components. Drive out the bipod pin as shown, and remove the bipod (don't lose the pin).

To remove the front sight, first undo the nut that faces the shooter using an 11/32 socket. Only having done that can the muzzle facing bolt be unscrewed (using the same tool). Having done that, tap out the sight using a non-marring punch.

First we have to remove the old receiver stub. First, the barrel pin is removed - either by the use of the press, or drilled out (as I had to do) if it just doesn't budge. Don't worry about saving the pin, because we will be using a larger one anyway.

Once the barrel pin is removed we attempted to unscrew the barrel with the hopes of saving the front stub. Alas, it would not move. With much work still remaining, it was decided to make a relief cut in the front stub and wedge it with a chisel to get the stub to split and let go of the threads.

Cut a slot in the lower side of the stub, as shown. Optionally, you can put a piece of metal like you see in the picture to prevent the cutting debris from entering the gas tube. Stop cutting once you see the threads showing. If you've cut fairly deep but the barrel still doesn't turn, take a metal chisel and drive it into the slot, so as to force the stub to split along the slot. This is safer that trying to cut all the way through.

Here is our improvised barrel-vise. Since the fit of the aluminum half-moons to the barrel wasn't too great, we used a press rather than a vise for applying pressure.

The stub was then unscrewed with an 18-in crescent wrench. You can see all the cosmoline that was trapped inside once it is undone. These kits really weren't shot at all.

The barrel stub can potentially be used in a re-weld by welding up the seam, and chasing the threads with a tap or on a lathe.

Now, we will turn off the barrel threads. First, grip the muzzle end of the barrel in a 4-jaw chuck, as such. Minimize the run-out but don't go too crazy because we will be redoing it again at a different spot on the barrel.

The breech end goes in a livecenter, however be careful -- the contact area is uneven because of the extractor cut, so make light cuts. This is where we will ultimately adjust our run-out, as shown. Try to get under or equal to 0.001".

Here is a pic of the entire setup.

The inside of the DSA receiver barrel hole was measured at 0.960". The diameter of the barrel in the area without threads came in at 0.965". As a first step, turn down the threaded area until it matches with the flat (i.e. down to 0.965"). Having done that, I chose to removed the last few thousandths by hand. My final barrel diameter was 0.962", which gave a good, workable press fit. It will become important not to have galling-inducing tightness in the next step where we press in the barrel using a 12-ton HF press.

Apply plenty of anti-seize and work the barrel in by hand just a little to make it stick.

Check alignment by putting in the gas piston.

Now, here is the tricky part. The barrel-receiver combination is very long and does not fit in an unmodified HF-12 ton. To get it to fit we used the following trick. A steel plate 0.5" thickness is positioned over the bottom cross bar as shown. To keep it from tipping the arbor blocks that came with the press were used to support from front and back. This does put quite a bit of pressure on the bottom cross-member and it begins to bend when pressure is applied. Thus, we cannot handle a whole lot of pressure with this setup (although a later modification of this setup performs much better -- see below). It is for this reason you want the interference fit between the barrel and the receiver to be workable. Push in until close enough to do headspace measurements. (There is a penny between the ram and the muzzle).

This is your last chance to check sights alignment. Put the receiver-barrel assembly onto a flat surface and rock it back and forth. This will let you feel whether your front and rear sights (well, flat surfaces, really) align. Correct by gripping receiver the a vise and turning the barrel with a large crescent wrench, or if that doesn't work, push the barrel back out and try again. Pushing the barrel back out is an easy setup (unfortunately I forgot to take a picture) and all it requires is a long enough bar stock to reach the breech while going through the receiver channel. Don't forget to put a penny on the breech face when doing this.

Ok, so now that we've gotten close to headspacing and we want to take a measurement, we notice a few nasty surprises. The first one is the shape of the extractor. Compare new vs. original.

The problem arises because the bolt begins to rise as it clears the extractor, jamming itself into the bolt carrier.

 This is the area of the bolt that is causing the interference.

At this point there are two options -- remove extractor from new receiver, file, put back in. Or we could remove the offending material on the bolt, which is what we chose to do. The removal was done with a dremel cut-off wheel.

The second nasty surprise is the engagement of the bolt lugs. One of the bolt lugs engages the receiver, while the other has a sizable gap on the order of 0.100". First, cover your bolt lugs with dykem, then check receiver fit.

Well, actually before you do that remove the extractor (don't have any pics of the process, but all subsequent work the bolt that you see below was done with the extractor removed).

You have to apply pressure to the bolt (from the side of the breech) with one hand, and try to move the lugs with the other. The solution seemed simple -- just remove enough material from the "long" lug for both to make contact. However, you only get one shot at this operation. Reason being, is that the barrel cannot go into the receiver beyond a certain point. This point is such, that it is possible to remove enough material from the lugs that your bolt is permanently too far back, and to fix the problem the receiver would have to be bored in a lathe to compensate. I came dangerously close to this situation. The filing is tricky because of the acute angle and the hardness of the material -- so it is easy to get carried away. I ended up cutting the long, the 'other' long and the long again. Here are some pictures of the process.

Oops, too far.

Final length of the lugs. This is pretty much the limit of the shortness.

Now, we can continue with the pressing in the barrel and setting the headspace. Turn the receiver-barrel upside down of the previous set-up and insert the go-gauge and the bolt/bolt carrier. Note the advanced setup illustration -- the red lines represent pieces of steel, preferably bar stock. In a pinch any two roughly equivalent pieces can be used - I ended up using a crowbar and a piece of round bar stock (sorry, no pic).

Now press the barrel until it stops. Let me emphasize, that you do not want to put lots of pressure on the go-gauge while it is in the chamber. Just push until the barrel stops, do not apply any more pressure. If you don't you may damage your gauge and will have a fun time trying to unlock the bolt. Now, disassemble and check with the no-go.

If you look closely at the pictures, you will notice that the no-go leads to a partial lock of the lugs rather than none at all. Perhaps this is due to the locking system, which is wedge-based in its nature. It was the same with both of our kits. Pushing the barrel in further prevented the go-gauge from locking.

Check the sights alignment with the 'table test' again, and you are ready to drill the barrel pin hole. Level the receiver along the x-axis. Note the rough surface that is given by a park finish. Center on the hole, and drill.

The new barrel pin that came with the trigger group measured 0.187" (3/16), so we drilled with a 0.184" (as measured, nominal size = 3/16) drill. The hole was clean -- completely covering the previous barrel pin groove. Unfortunately, for whatever reason, the resulting fit gave no interference at all. This was temporarily overcome by knurling the barrel pin. (Note: we used a generic enco brand 0.250" dowel pin for this, as the pin that came with the FCG was really hard and did not knurl). We will redo the barrel pin in the future using a larger size pin.

Next time, we will cover the modification of the lower and the fitting of the FCG group. Stay tuned.

Semi RPD Build [Part 1] - The Bolt Carrier

Introduction

This series of posts will deal with building a semi-auto RPD conversion from 1960s Polish kits (intact barrels). We will be using DSA receivers (though, I understand that they went off the market in early 2012) and trigger groups from Project Guns. This is a long project which was done over a week of full time building and the writeup will be split into parts, each dealing with a particular aspect of the build. I will also post some information on demiling barrel stubs from barrel-less kits.

This is by no means the first RPD build out there. In fact, there are several descriptions (of varying detail) that can be found online. The most informative and extensive is the information provided by Pirate of Project Guns, to whom we wish to express our acknowledgment. While a determined individual can find all this info from bits and pieces scattered around different sites and forums, there are many inconsistencies in the reported dimensions. Furthermore, while some aspects of the build are covered many times over (e.g. bolt carrier mods) others are not covered at all (e.g. fitting the trigger group). Thus, it is our goal to address the mentioned issues by posting our own build here, free of charge, start to finish - with all the small details.

Part I. The Bolt Carrier.


The build sequence begins with modifications to the bolt carrier, as it will allow us to start fitting parts together in the receiver. The fitting of the bolt, the charging handle and headspacing will all depend on a functioning bolt carrier. Another note that should be made here is that the first thing you want to do to the bolt carrier is to machine away the hammer island, so as to prevent full auto functionality before anything else is done. (Admittedly, this was an oversight on our part and we did not follow this order. Learn from our mistakes, do it the right way yourself.)

As this will be your first machining operation of this build -- make sure to check the alignment of your machine, in particular the alignment of the mill vise (unless your vise is keyed, but even then it would be a good idea). We will still align each part individually, but it helps to start with a fully square setup. Also -- make sure you've trammed your mill recently.

This is a pretty easy cut, but still take care to level the carrier with a test indicator. If you followed the steps above, it will probably suffice to level only along the x-axis. Here is the setup:

You will be removing 0.100" from the face to give the final measurement of 1.162" (see pic). Use a 3/8 in mill for this.

Here is the final product:

Now we move on to trimming the rails. See the pictures for illustration of the setup. Leveling is done on the two edges highlighted in red. Note that this is done before milling off the sear catch, as that area is good to grab onto for clamping the carrier in the vise.

Just to be clear, we are trimming the lower side of the rails -- see schematic above. Amount to cut is 0.025". This cut was done with a 3/8 end mill. Here is the final product. The other side is done in analogous manner.

Now, we do the sear catch. Once again, here's the setup, with edges to level on highlighted in red. You will be cutting away just enough material to widen the sear channel to uniform width.

After the cut -- using a 1/2 in end mill:

Next step is to drill the firing pin extension (FPE) hole. This is a bit tricky due to the setup, and you will need 4 123-blocks, the usual mill clamping kit and (this is the key part) a precision angle with bolt slots (or something similar). We barely had enough room on our mill to clear this setup. If all else failed, the gas piston could have been taken off, but we really wanted to avoid that. The dimensions for the hole location are given below along with the location for the cross pin (as the picture states, these numbers are from the Project Guns website).

Here is the setup for drilling the FPE hole.

First drill with a 0.185" bit (#13). Then finish with a 0.189" reamer.

As you can see, the FPE ends up being a bit low with respect to the firing pin, however the contact area is sufficient for perfect functionality. Furthermore, positioning this hole higher is a risky proposition because there is not much material as it is between the FPE hole and the top of the bolt carrier. Although, if doing it again, I would move the center up maybe 0.005 or 0.010".

Having done the FPE, we can now do the cross pin. Here is the setup.

Again, drill to 0.116" bit (#32) then ream to 0.1247".

My markup ended up being a thou or two high, so I had to file a little bit of material from the FPE (see pic). This steel is extremely hard to file by hand, but a sandpaper dremel wheel does the trick nicely.

Here is the final product

Make sure the FPE is completely free to move. With this, we will move on to the barrel, bolt and headspacing.