FN30 Semi Build

Inventory your parts set here.

A Note For Anyone New to Building Your Own Belt-Fed:

I'm documenting my FN30 build in the hope that it will give others something from which to take their bearings. While I have built several semiautomatic weapons (1919A4s, AK-47s, Sten) I am not a gunsmith. There is no shortage on the Net of advice given by those with little practical experience and no formal training. Here is yet another one. Keep in mind that the forces involved in a Browning belt-fed are considerable, and while I applaud anyone willing to take on such a build, I can't stress strongly enough that you should take care as you work, and please make liberal use of the many qualified people out there who's advice and supervision can make the difference between a truly fun build resulting in a historic weapon you can pass down to your grandkids, or an opportunity for a very serious injury. PLEASE read the entire tutorial before you start.

Using the link in the upper right, check your parts set against the one shown to make certain that you have all of the necessary parts. Go over each part and dress any burrs or dings you find, then trial fit the main parts to see that they go together nicely and to get a sense of how the components fit. These kits seem to have taken a fair beating and you may have to do some work to get the parts to fit properly. In particular the sight base is a tight fit under ordinary circumstances and if dinged at all can require some TLC to get it to slide into the top plate sight rails. As you will see, there seems to have been some fitting in the manufacturing process, and since these kits are assembled from random parts and are not necessarily matched as originally built, we will have to tweak.

In order to do the test fitting you will need to locate the two alignment splines. One of these is shown above. The splines key the side plates to the trunnion (left, red arrow). If you have examined a side plated from a Russian Maxim 1910, you are familiar with this feature. Some kits have only one of these so you will have to make a second. If you are missing these altogether you will need to clamp the side plates to the trunnion with the slots aligned, then take the cross section measurements, make the splines, reassemble the side plates and trunnion with the new splines in place. Using the rivet and trunnion holes in the plates as a guides profile the splines by redrilling the holes or using a Dremel tool or similar to shape them.

When test building a receiver I use clamps to apply some force to the top and bottom plates, and one at the rear holding the side plates against the back plate which serves to align everything.

Everything seems to be in order with the receiver parts, so I check to see if the trigger pin will fit the hole in the side plate

The trigger pin hole is a bit too small, so I carefully ream it until the pin just slides in. Not too much - you don't want a loose fit.

On the Original right side plate there is a bevel.

Use the left side plate as a guide and carefully file a 45 degree bevel.

You are going to need to use transfer punches to mark the centers for drilling the rivet holes in the side plate. I start with the belt holding pawl bracket (BHPB). Align the BHPB with the front of the plate and the edge of the ejection port as shown, clamp it firmly in place, then mark the three rivet holes. Be certain that the BHPB does not creep when you punch each center.

After the BHPB is done, assemble the receiver and clamp everything firmly in place. Use the back plate to help keep everything in line. Mark the centers for the top and bottom plate rivet holes from the outside, and then lay the assembly on its right side and mark the rivet holes from the left side by inserting the transfer punch through the trunnion and sight block holes as shown in the photo at right. Leave the two transfer punches in place in the two holes indicated by the red arrows.

Note that the pintle holes in the trunnion and left side plate do not line up. This suggests a mismatch in the original parts, as well as being evidence that even with standardization of parts, FN tweaked each gun individually.

There is also some work to be done on the left side. Using the trunnion as a guide, I relieve both side plates using a Dremel tool with a small grinder. The transfer punches I left in place keep things in line.

I test the trunnion sleeve that came with my builders kit and it passes through the trunnion and both side plates At right is a view from the other side. A ring will eventually be placed over this end.

Drill the holes using the appropriate drill bit. The lower front rivet hole is too close to the spline groove to drill, so using a 1/16" drill bit I make a pilot hole, then flip the plate and drill to the correct size form the other side.

The top and bottom plate holes are drilled using a 3/16" x 60 degree countersink, but I don't bevel the holes yet. I clamp the side plate to the bottom plate, carefully lining up the back edges of both. I then drill through the first hole, drilling through both plates at once, using a #7 (13/64") bit. I slip one of the rivets in this hole to keep the back edges even and then run down the line. Having started with the smaller 3/16" makes it easier to drill, and leaves enough material to allow for the inevitable misalignment. You can skip the 3/16" step if you want, I just prefer to do it this way.

3/16" x 60 degree countersink. This is what I use for my 1919 builds, so I started with this. It doesn't seem to be an exact fit for the rivets.

The builders kit I bought from provided rivets that are a tad bigger in diameter than the originals, which allows for drilling the holes larger, removing any dings and wallows creating during the demilling process.

These are the rivets that came with my kit. I laid them out and figure out where they will go.

Right side plate showing rivet locations.

Rivet the left belt holding pawl bracket (BHPB). The BHPB on my left side plate has domed rivets, and the rivets I had were not long enough to dome, so I set them flush. I may set the left BHPB rivets flush to match.

Lay out the top plate and cocking block and clean everything up. DO NOT INSTALL the cocking block yet, or you will block access to the rivet holes on the right side. The block will be added after the side plates have been riveted on.

My left BHPB. I have seen these flush as well. The 1919 BHPB rivets are flush.

There is a top plate rivet hidden by the sight plate, show at left, so rivet the left side plate first, then the sight plate, then the right side plate.

I used for the #5 rivets (as shown on the chart) for the sight base. They may have been intended for use installing the cocking block to the top plate. In any case I came up short two rivets needed for the sight base plate (may have gotten lost when I accidentally scattered the rivets across the shop floor) so I made two.

To make a rivet I took a mild steel rod (3/16"), tightened it in a pole vise, then slipped the cocking block over the end with about 3/8" protruding. Using a small ball peen hammer I mushroomed the head and worked it into the bevel in the cocking block hole. I ground the excess until the rivet head would seat flush, leaving a nice head as shown at right. I left the rivet long as it was a loose fit. When set, the rivet will expand to fill the hole.

I used one of Tanker's excellent bucking blocks. Surprisingly, I don't actually own one. There always seems to be one handy when I am doing a build. This one is courtesy of MCP. I use hack saw blades to protect the bucking block from being dimpled by the rivets. You can see one peeking out (green arrow).

Above: A line of rivets showing the stages of setting. Starting with an inserted rivet (1) I use a 1 1/2 lb. hammer to flatten the rivet (2) and then a shallow doming tool to round the edges (3).

With the top and bottom plate rivets done, it's time to clean up the inside of the receiver.

Using a large flat file, work the rivets until they are flush with the inside of the receiver. I always grind the teeth off of one edge of a flat file so that I can work up against an adjacent surface without marring it.

Left & above: The cocking block, shaded green, fits under the top plate as shown.

I used a flat file to dress the excess rivet material.

I clamped a length of 1" square stock (blue arrow) in the pole vise, and slipped an old tool steel planner blade under the rivets (red arrow) before clamping it. I used an air hammer with a flat round bit to work the rivets down. I didn't want to be swinging a heavy hammer around the sight base rails.

To back up the bottom trunnion rivets I put a piece of 1" bar stock in my pole vise, and used a planner blade to protect the softer bar.

I used a "C" clamp as shown, passing the lower pad through the pintle spacer hole.