This tutorial will cover building a semi auto Bren gun. The kit being use
is a MK1, and the process is the same for a MKII. Since the receiver cuts
vary your weld up may differ slightly from this tutorial.
The internals will be machined first, eliminating the FA features and
making the clearance cuts for the denial areas that will be built into the
receiver.
We will start with the bolt and bolt carrier. The first step is to
disassemble the bolt, remove the firing pin and extractor. Then remove
the bolt ramp assembly from the bolt carrier and degrease all the parts.
The Bolt: Clearance cuts are made with a 1/2″ end mill in the front of the
bolt on both sides to the depth of the existing reliefs, this will allow the bolt
to clear the denial pins in the receiver.
Bolt Carrier: Machine off 1″ of the guide rail on both sides to clear the
denial areas in the receiver.
Bolt Ramp: Machine a 3/16″ slot 5/16″ deep for the semi auto firing pin.
Also machine .100″off the face that contacts the FA firing pin.
On the bottom machine off the FA sear as shown in the second photo.
Lower Receiver: This needs to be cut, and the front will be welded to the
receiver. The area in front of the safety needs to be filled with weld to
prevent the safety from being pushed forward to the FA position.
This completes the most important modifications to the internals, Now
the receiver can be prepped for reconstruction.