Semi RPD Build [Part 3] - Lower, FCG

Part 3.

In this post we will cover the modifications to the lower and FCG. The FCG was purchased from Project Guns.


Disassembly.

Firstly, strip all the wood from the lower. Knock out the trigger pin first, and remove the safety and the safety spring.

  

The lower will need to be cut to length.

Here is a blueprint that we will use for the next few steps. Refer back to it for dimensions as you read further.

First, mark the location of the trigger bar channel cuts. Then some material was taken out from the trigger bar channel using a dremel, so as to have less material to mill out. Be extremely careful when doing this. There is nothing wrong with milling out the entire area either, it just takes a little longer. 

Using a machinist square, mark the length of the sear pocket and the overall lower length. You can see the final markup in the second picture (the front has already been sawed off).

Now, we are ready to do some mill work - clean up the front of the lower, mill out the sear pocket, mill out trigger bar channels, mill out the round trigger bar relief. Hold the lower as shown below, and align the z-values along the x-axis, also double check consistency of y-values as x-axis is traversed.

The trigger bar channels are cut using a 1/8 carbide end mill. To start the cut - touch off on the inside of front cavity (the longer narrower one) and cut (in several passes) towards the stock. 0.125" is a sufficient width for the channel and further widening is not necessary. 

The sear pocket is milled out using a 1/4 end mill. For trimming the front, we used a 3/8 mill, but any large-enough cutter will suffice.

 

The following picture, shows the (almost) finished product. There is still one more cut remaining - it will be done shortly.

We can now start fitting the lower to the receiver. Using a small file, round the front of the lower as shown in the picture.

Additionally, these areas of the receiver will need to be rounded to mate with the lower.

The charging handle also needs a little filing. The areas that need to be filed are circled in red in the following picture. Also, give the flat area on the top a little work with some sandpaper.

Here is the fitted assembly.

Having fitted the lower, we can now drill the pin holes for the FCG. Here is a blueprint that we used. It is based on the blueprint was originally included with the FCG, however some values have been updated to fit the included parts. Refer to this picture for dimensions as you read the description below.

Here is a picture of the FCG itself, which includes: new rear pin, new barrel pin, screws for installing the drum bracket, trigger spring, striker, striker spring, FCG pins (there are 4, though you will only need 3), trigger bar (contains sear and disconnect), trigger bar spring, firing pin extension and firing pin extension cross-pin.

If you install the trigger bar, you will notice that it rubs against the lower in the area shown.

To accommodate this, we will mill out a relief cut in the lower. This is an inexact cut, as it is difficult to fit the ball end mill into this particular area of the lower. Using a 1/2 ball end mill, cut as long of a cut as you can manage, down the centerline of the lower. The total depth of the cut is 0.015".

Here is a picture of the trigger bar installed. Make sure it is free to move back and forth.

The trigger needs to be modified, in order to accept a trigger spring. This is accomplished by making two cuts using a 1/4 end mill. The first cut, 0.050" deep, starts from the center of the existing trigger pin hole and proceeds in a direction that is perpendicular to the nearest edge. The second cut, 0.100" deep, also starts from the center of the pin hole but proceeds at a 45 degree angle to the first. You can see both cutting directions marked in this setup picture. Center on the hole and proceed in light cuts (~ 0.010" per pass) from the center outwards.

The first cut is done.

We now, realign the trigger along the next cutting axis. Note the original mark is no longer visible, so we draw a new guide.

Center on the hole and proceed in small increments, as before.

Here is the finished trigger after some cleanup with a small file.

There is one more operation that needs to be applied to the lower - we need to drill out the return rod hole to 3/8. The setup is shown in the picture below. We use a machinist square rather than a test indicator for alignment due to the difficulty of getting the latter into the area of the measurement. The entire setup is extremely vibration-prone, since we're holding the lower by a (thick) piece of sheetmetal. For this reason it is very important to drill the hole in increments and at a small feed rate. We drilled first to 11/32 and then to 3/8.

Finally, a chamfer is applied using a scraper - a ground down triangle file.

Installation of the trigger and its spring reveals that the spring is rather long and will need to be cut to length.

Mark the area where the spring touches the lower, and using a dremel cut-off wheel make a groove just deep enough to hide the spring wire. Cut the spring to length -- careful here, cut it purposefully long and then trim to fit.

Without going into chronological detail as to how these locations were identified, I will simply show you which areas of the trigger bar were making too much contact with the trigger and its spring. You will need to file these areas just enough to clear the binding. (You will also need to file the trigger itself, so don't focus on the trigger bar alone.)

In the following picture we can see the areas of the trigger nose that will need to be fitted.

To fit the trigger, you will need to remove a significant amount of material from the top of its nose, as well as the sides of the nose (see pic below). Additionally, you will need to polish these areas to a shine, in order to ensure a smooth function of the disconnect. The entire thing must operate smoothly, no matter how hard you pull the trigger.

Here is a picture of the finished trigger.

Here is a picture of the fully-assembled FCG.

And here is a video of the FCG in action (sorry about the blur :) Stay tuned for the next post, where we will put the rifle together and do some test firing.



PS. In case of Bubba, break glass -- a short story.

Without going into details as to how this occurred, other than to say -- no it wasn't me, the lower which I had been painstakingly working on for so long was scarred by Bubba's favorite weapon - the dremel.

This was very upsetting, as the whole thing was almost finished, only to receive this horrible mark. I did not want to weld, as the location of the gauge was very close to the FCG pin hole... but how else to fill this void? And then it hit me -- soft solder. The location of the gauge is on the bottom of the lower (not a critical area) and it will be finished by parking followed by dura-coating, so there really should be no difference in the finish once the paint is applied. Here is what to do. Apply a generous drop of soft-solder to the damaged area.

Using a MAPP torch heat the solder from below, otherwise the gas pressure ends up blowing away bits of the solder. Once it melts, let go of the heat, and check for air bubbles and small voids in the filler. If necessary apply another layer of solder and melt it again. Once satisfied, take a hand scraper (like the ground triangle file from before) and scrape the bulk of the solder off. Follow by a sandpaper polish, and you will have something that looks like this:

Damage undone. Nice!

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.