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Build your own Bren gun! he's gathered the parts and supplies (7/20 issue); now Matthews starts by crafting the U.S.-made parts that will be required to make a legal semi-auto Bren.

* Part 2--Compliance Parts


Several of the original full-auto Bren parts will be reconfigured as semi-auto parts. Some parts will be extensively modified while others will only need slight modifications. Many parts will function the same as before but be modified to fit in the semi-auto receiver.

You may wonder why these parts need to be modified. The answer is simple: because the BATFE says so! The BATFE approval letters approve a certain method for making a legal semi-auto Bren gun. Any significant change in the approved design may be illegal.


While exact dimensions are not specified in approval letters, the basic operating system must be maintained. If you want to go your own way on this project, contact the BATFE and get the new methods approved.


You can't make a gun that is not importable entirely Out of foreign parts. You have to combine it with enough U.S.-made parts to qualify it as a domestic product. I made all the regulated U.S. parts myself, namely the receiver, striker, trigger, gas piston, pistol grip, magazine floorplates and op rod. The need for a U.S.-made op rod is debatable due to how parts are named. Many sources call the rod that drives the bolt carrier forward an op rod. Op rods are on the BATFE list of regulated foreign parts, however none of the approval letters I read called this part an op rod.


A new op rod/recoil spring, assembly will be made so either way it will be a U.S.-made part. These six or seven (if the op rod is counted) parts will result in a regulated parts count of 10 or less to maintain legality on the U.S. compliance issue.


I started by making the easiest part first by duplicating the original pistol grip. You could make it any shape you please. I started by cutting out a chunk of rectangular wood out of an old military gunstock. I traced out the shape, of the original grip and then cut out the shape with a bandsaw.


I drilled the grip screw hole through the rough cut grip. This hole is at a 90[degrees] angle so it is easy to drill right. I cut the recess in the top with a milling machine, but you could also form it with a router or Dremel tool.

To complete the grip, I simply shaped it to original form with a rasp and sandpaper. Once it was completed, I stamped it with my initials and USA to identify it as a U.S.-made part.



Next, I made a new trigger. It is identical to the original trigger; its whole purpose was to add to the U.S. parts count. I drew the outline of the original trigger on a piece of O-l tool steel that was the correct thickness. You could use 4140 or any other high strength alloy steel.

I cut the shape out with a bandsaw and then used a hand grinder to shape it like the original. Once the outside shaping was done, I stamped the part with my initials and USA to identify it as a U.S. part. I then clamped the old and new triggers together and used the original trigger as a drill guide to drill the new pin holes.


I cut the slot in the top with a hacksaw and then filed it to final shape. Since my O-l tool steel was heat-treatable, I heat-treated the trigger to increase its strength.

While many hobbyists simply guess on temperature, there is a simple way of getting the right temperature before quenching the part. A liquid known as Tempilaq, available from Brownells, is available in several heat ranges.

Since the correct heat-treat temperature for O-l tool steel is 1475-1525[degrees], I obtained 1500[degrees] Tempilaq. Simply brush it on the part and then heat. When the applied dried Tempilaq melts and flows out, the right temperature has been reached. The quenched hardness of O-l is about 60 on the Rockwell C scale, too hard for this part. To temper the part to a less brittle state and lower hardness, I floated the part on some molten lead in a bullet casting pot. I used 700[degrees] Tempilaq to gauge temperature and annealed the part for one hour. This brought the hardness down, to about 44 Rc, about right.



Magazine floorplates add U.S. parts count and are very easy to make. Using simple sheet metal forming techniques will let you make them for less than $1 apiece. I used 4130 sheet steel in .035" thickness. I obtained 6x12-inch pieces from Aircraft Spruce ( for a few dollars each.

I made a pattern for the floorplate blanks by unfolding and flattening an original. I laid it on the sheet steel, traced the outline and then cut out the floorplate blanks with a saw. I cleaned up the rough-cut blanks with a hand grinder.

I scribed a fold line on the sides in the same location as the originals. Then I clamped the blanks in a vise at the fold line and used a hammer and piece of steel to bend the edges over to 90[degrees].


To fold the 90[degrees] edges over to grip the lip on the bottom of the magazine body, I just made a forming guide out of a piece of the 4130 stock. This guide was sized to represent the shape on the bottom of the magazine.

I placed the guide on a partially formed blank and hammered the edges over. The guide was tapped out of the floorplates and then I used a thin putty knife to pry the bent edges up a little for an easy fit on the magazines.


The end tab was bent to duplicate the angle of the originals and I drilled a hole in the bottom, just like you'd find on the issue mag. The floorplates were then stamped USA.


The U.S.-made gas piston will duplicate the original. The fabrication of this part is fairly easy. The one process that may intimidate some amateur machinists is cutting square threads. Rather, than using standard V-threads, the Bren designers opted for square threads, since they take impact forces better than V-threads.


The threads are at a pitch of eight per inch. With a proper lathe setup, these are really not that much harder to cut than regular threads.

Square thread dies are expensive and hard to find, so it's a lot easier and cheaper to cut them on the lathe. The original Bren gas piston is made from stainless steel. I did not have any suitable stainless in stock, so I opted for a piece of precision ground O-l drill rod that I did have.


O-l does have some degree of corrosion resistance, plus I figured I would spray it with GunKote when finished. The first thing I did was to turn a stub, the same size as the original, for the threads. The gas piston is supposed to be a loose fit in the carrier, so don't think the original is worn out because it is loose.

My first step in cutting square threads was to grind a square point at the front of a cobalt tool bit. This point was the same width as the square thread grooves. I set the lathe up for eight threads per inch and set the spindle on the lowest speed.

Unlike V-threads that are cut at an angle, I cut these threads at 90[degrees]. I then started to cut the threads at a rate of a few thousandths of an inch per pass. I used plenty of lube and checked the size often. Cut the threads deep enough to have the slightly loose fit like the original.


Once the threads were cut, I drilled a shallow centering hole in the end to support the workpiece while I turned the remainder of the part. With the part supported between the chuck and live center, I turned" the rest of the part to approximate the original.

Make sure you duplicate the piston head diameter precisely, since the gun won't function properly if the piston is not sized properly I removed the part from the lathe and stamped it USA.

The original Bren gas piston is hollow. If left solid, it will add extra weight to the reciprocating mass of the carrier assembly and possibly cause functioning problems. Drilling an 8-inch deep hole precisely, even on a lathe, is no easy chore.

Drill your hole undersized and check that the bit hasn't run off center before you drill up to full size. You don't want to drill through the side of the part you just spent all that time making!

I actually drilled my hole under size by about 1/32" just to be on the safe side. This resulted in a gas piston that weighed about a half-ounce more than the original, but I figured it would still work OK.

The last machining operation was to cut the groove for the crosspin that locks the piston in place. Since this was a heat-treatable steel. I hardened the head of the piston for wear resistance. I used 1500[degrees] and 700[degrees] Tempilaq to get the right hardness and temper.


The new semi-auto Bren required a spring driven striker, so the op rod/buffer system had to be redesigned. The system I came up with is of my own design and is not compatible with any purchased semi-auto Bren parts.


I needed two springs behind the bolt carrier, one for the carrier and one for the striker, operating independently. Unfortunately, these need to occupy the same limited space behind the bolt carrier.

Add to this the fact that they would be long, skinny springs that would need to be supported so they didn't kink. I tried several setups and scratched my head for several days with no success.

Finally, one night while lying in bed unable to sleep, the answer just popped into my head. What I came up with was a telescoping tube system that featured an internal spring for the recoil buffer and an outer spring for the striker. This assembly would take the place of the original op rod and recoil spring and would count as a U.S.-made part.

I will explain how I made this part but there is one variable that may affect your results that is beyond my control: spring strength.

While I will specify the sizes of springs I used, not all springs of a given size are the same strength. Due to spring temper and the material used, the same size springs can vary considerably in strength. I obtained my springs from McMaster Carr Co. of Cleveland Ohio (

I started with some 4130 seamless tubing for the telescoping tubes. I got it from Wicks Aircraft of Highland, Ill. For the outer tube, I used 6 3/4 inches of 1/2 (.500" O.D.)x.065" wall thickness. This tube had an internal diameter of .370". For the inner tube, I used 10 1/4 inches of 3/8" (.375") x.028" wall thickness. This tube had an internal diameter of .319".

Do not be surprised if your tube diameters vary somewhat. To get the .375" O.D. tube to fit in the .370" I.D. tube I had to ream out the larger tube and polish a couple thousandths off the smaller. You need to have a couple thousandths running clearance between the tubes so they slide together easily.


The next part I needed was a recoil spring. The spring I bought was 5/16"x.048" wire at 12 coils per inch (#9662K42). I used 18 inches of this bulk length spring. This spring must slide easily inside the small tube. If it doesn't, take a couple thousandths off its diameter by sanding it with it supported On a rod spinning on a lathe.

The part of this spring that fits in the larger tube must be supported with a guide rod to keep it from kinking when it is being compressed during recoil. I made a short bushing with a thin lip on the end that would be brazed into the end of the larger tube. Overall length of this bushing was about 3/8".


This bushing had a hole in it for a 7/32" guide rod that would be made from heat-treated music wire (available at hobby-shops). The spring must slide easily over this guide rod, so you may have' to alter the guide rod's diameter for smooth operation. Be sure to round the end for smooth operation. The guide rod must be centered in the tube to operate correctly so pay attention to this when brazing (or welding) the bushing or rod in place.

You'll need to make a cap with a small stub for the front of the smaller tube. This stub will fit in the back of the bolt carrier. I made the stub .240" diameter and 5/16" long. The bottom of the cap was sized to fit in the end of the tube and was only about 1/8" long.


With the spring installed, the tubes should slide together easily when compressed. To prevent this close fitting telescoping assembly from functioning as an air spring, drill vent holes in the ends of the tubes to allow the air to escape and refill quickly. I drilled six 1/8" vent holes in the last 1/2" of the large tube and three 1/8" holes in the last 1/4" of the small tube. These were spaced at about 120[degrees] intervals around the circumference of the tubes. When in use, the tubes and springs should be lightly lubricated.

While it is not needed yet, I will specify the size of spring I use for the striker spring that will slide over the smaller tube. I used a .480"x.041" wire at eight coils per inch (#9637K81). These springs are 11 inches long but will be stretched out to 13-15 inches later. The rear of the spring will bear against the large diameter tube and the front will fit in the rear of the striker.



The lower receiver needs to be modified to fit the upper receiver. The fire control group must also be converted to semi-automatic operation. The first operation is to grind off about 1 1/4 inches of the rails on the front portion of the lower receiver. These rails fit in grooves in the upper receiver and allow the upper receiver to slide rearward about 1/4" during recoil.

The new semi-auto Bren receiver will have shorter grooves to prevent a full-auto lower from being installed. I simply ground the rails off smooth with the sides of the receiver with a small hand grinder.

Since the new semi-auto Bren was going to be striker fired, the original fire control group (except for the replaced trigger) could be utilized if modified for semi-automatic operation.


The original Bren trigger group provided both full-auto and semi-auto operation. The full-auto feature must be permanently removed. The Bren trigger group consists of the trigger, disconnector, sear, safety/selector lever and associated springs.

The way the original group provided both firing modes was very simple. When the trigger is pulled, it pulls on the disconnector, which engages the sear. There are two protrusions on the disconnector and two engagement surfaces on the sear.

The bottom portion creates full-auto fire by pulling down the sear and keeping it held down till the trigger is released. Semi-auto fire is created when the selector/safety lever allows the disconnector to engage the top portion of the sear.

In semi-auto mode, the disconnector rides higher in the lower receiver. As the bolt carrier reciprocates, a protrusion on the bottom of the carrier assembly disengages the disconnector from the sear. Since the lower full-auto portions of the sear and disconnector are not needed, they will be removed.

I ground off the full-auto catch on the bottom of the disconnector flush with the bottom of the disconnector. The lower engagement surface on the sear was ground away. These parts are rather hard, and files may not be hard enough to remove the surfaces. If not, use a Dremel tool with grinding wheels. When removing the lower engagement surface on the sear, be careful not to damage the upper semi-auto section, as they are very close together.

To smooth disconnector operation, the top surface of the disconnector was radiused slightly. This will allow the bolt carrier and striker to slide over the disconnector easily. Do not change the Overall height or it will not function right.

Before the new U.S.-made trigger was installed, I removed the trigger spring from the disconnector and clipped off 1 1/2 coils to reduce tension on the trigger and disconnector.

To prevent the safety lever from rotating into the full-auto position, I made a safety lever stop from a # 8 fillister head screw for a stop. I drilled and tapped a hole for the screw and installed it. I let a short section extend into the receiver and placed a small weld on the end to secure the screw permanently to the receiver.


To allow the new telescoping buffer assembly to fit in the buffer tube, I opened up the hole in the end to 1/2" to allow for clearance of the new buffer assembly and striker spring. The buffer tube can be removed for drilling by unscrewing. There is a-stiff spring inside it, so be careful when you remove the tube.

Next month (9/20 issue): Bolt and bolt carrier.
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Title Annotation:Bren light machine gun
Author:Matthews, Steven
Publication:Shotgun News
Geographic Code:1USA
Date:Aug 20, 2010
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