This build will be done with an ORF receiver, compliance parts set and
a VZ58 parts kit.
You will need a #26 drill bit / .147″ and a 4mm / .1575″ reamer for the
barrel pin hole. The reamer is available from MSCdirect.com
Part # 02296408 for about $14.00. It may seem like an unnecessary
expense, but it is the right tool to get the barrel pin to fit properly.
The head space gages are available from several gunsmith supply
vendors. You will need a Go gage at the minimum.
If you want to duplicate the original gray color it is available in Duracoat
The first step is to strip the barrel assembly then remove the barrel pin.
They are in there really tight and they will not be used, so the easiest way
to remove it is to drill it out. Use a good sharp bit with cutting fluid.
Next press off the receiver stub, they are really tight, so you may want
to just split it on the bottom with a hacksaw or dremel.
After removing the barrel, remove the rear sight. Use a small punch to depress
the sight spring and slide it out. Then remove the piston stop, it is the little
crescent shaped part in the bottom of the photo on the right. This needs to be
installed in the new receiver before the barrel is pressed in.
First we will prep the barrel by permanently installing the muzzle break / barrel
extension. I opted to silver braze it on with silver brazing paste it is a neat and
quick method. I used the silver brazing paste from SRA.
The parts must be clean and free from oil or grease, I gave them a quick
blast to clean them up and remove the paint from the sight base. The
paste is applied at the front of the threads and the muzzle device screwed
on. All excess was removed with a Q tip, this makes clean up easier.
Here is the finished product, a ring of braze can be seen inside the
muzzle break, indicating the process was properly done. A quick blast
cleans it all up. Now the barrel is legal length and can be installed.
The next step is to fit all the parts to the receiver and install the trigger
guard and sear spring.
The sear spring needs to be modified as shown in the photo, by cutting
off one of the arms.
Now the trigger guard and sear spring will be riveted to the receiver
using the 2 rivets in the compliance kit from ORF. The holes in the receiver
and trigger guard are to small. The front hole needs to be opened up to
.125″ and the rear one to .150″. It is a good idea to blast the trigger guard
before installing it, it will save a lot of time later.
Set the front trigger guard rivet first, then install the sear spring over the
rear one and set the rear one.
Now install the ejector and stake it in place with a center punch.
The next step will be to fit the fire control group. You will need the
safety and trigger from the kit, the sear bar spring and pin will be used
from it. I opted to modify and use the original pins as there are some
problems with the ones from ORF.
The other parts shown are the semi auto sear and pin, trigger and sear
bar, safety retainer and new pins. Check the safety and pins for fit and
correct as necessary.
Now we will prep the sear, the hole in the sear for the roll pin is to
small, the pin measures .093, so I use a .089 drill to open up the hole.
instal the pin as shown in the photo.
I decided to modify the original pins as the headless ones that come with
the Kit can shift causing problems with the FCG parts. Since the new
receiver is thicker, the slots for the retaining clips need to be re cut. I did this
in my mini lathe with a mini die grinder and cutoff wheel.
Next assemble the US trigger and sear bar using the pin and spring
from the one from the kit.
Install the trigger and safety / selector as shown, check for proper
operation and correct any binding.
Now install the sear and safety retainer and test the trigger / safety
operation. Correct as needed.
Now it is time to install the barrel. There has been some discussion on
a couple of the forums about barrels being to loose. The receivers were
made to fit a barrel with a diameter of .825″, the receivers were bored to
.8245″ to give a .0005″ press fit. If your barrel is to loose in the receiver it
will need to be addressed before continuing.
First the gas piston retainer needs to be fitted and installed, it can not
protrude into the barrel hole.
I purchased the receiver fixture from ORF for doing the barrel, it is worth
the $20.00 they charge for it. It properly supports the receiver during barrel
I prepped the barrel for installation by slightly beveling the first 1/4″ or
so on the belt sander, then polishing the rest with 600 grit emery to just
shine it up a little, you do not want to remove metal unless your barrel is
oversize, the target is a .0005″ press fit. Be sure to apply some grease to
the barrel before pressing it in.
The barrel will need to be aligned with the sight base at 90 degrees,
there are several ways to accomplish this, by marking the barrel and
receiver or by eye.
When using the jig the bolt can not be installed to press the barrel onto
a head space gage, it appears that pressing the barrel in until it contacts
the shoulder in the receiver will get it set. ORF includes some .004″ shim
stock in the semi parts kit and recommends putting this under the bolt lock
to leave a small gap so the bolt can settle in after firing and keep the head
space in range. After pressing in the barrel check the head space with a
gage and an empty round. When it is correct proceed to the next step.
Now drill the hole for the 4mm barrel pin. Start with a drill bit about
.010″ smaller than the pin. A .147″ bit is what I used. Be sure to use a
sharp bit and some cutting fluid to get a clean straight hole. After drilling
chamfer the holes on both sides.
It is extremely important to get the barrel pin to fit properly, so a 4mm
reamer should be used. Be sure to use cutting fluid with the reamer to get
a good clean, precise hole.
Now install the top cover retaining pin and plunger, the plunger rides on
the sear spring inside the receiver.
All that is left is to install the hand guards, pistol grip, stock, bolt carrier,
bolt and top cover.
After final assembly I found the trigger is to long and hits the trigger
guard. It also is not shaped very well, so I made a few corrections.
I shortened the trigger by 1/8″ to give it clearance on the trigger guard.
Then I used a sanding drum in a dremel to round the sharp edges. Now it
has a lot better feel to it.
There was a problem with the striker hanging up on the rear
face of the sear, I rounded the edge to prevent this from happening
and it got the setup working properly.