New M70 barrel install tutorial

[Rahatlakhoom]

This is the mil-spec Yugo barrel from Centerfire. It comes in the white.





I had my new barrel hot-blued before this installation proceedure.
I have never attempted this before.
It was difficult to know where to start, but,
It's arbitrary at this point.

I took a long straight edge and drew a
rudimentary centerline with a silver sharpy. It's not perfect, it will serve
as a visual start point on the chamber cut-out. You will note from your
old chamber the orientation of the extractor cut-out. It looks to be
in the 12 and 3 o'clock position. Draw a centerline on the old barrel
across the barrel pin slot and RSB slot. You will note that
the top cut-out corner is left of 12 o'clock by just a smidge.
Scribe this difference onto the new barrel proceeding from
the centerline mark.








After you have established this point, hold up your old Yugo
barrel chamber to the new one and scribe or mark the boundary
line of the lower corner to the new barrel.






Next, to establish the depth of the extractor cut-out area.
I used my calipers to make the actual scribe mark.
.050" is an average I took from 4 different yugo barrels.
There are variations on this cut between barrels as much as .015"
So, relax.




I decided to remove the metal with a tungsten-carbide burr bit
in a dremel. This took it out really fast.
Get the barrel end set up for your own comfort zone so you can
maintain control of the removal process. Take it down almost to the
scribe mark and then leave the rest for file work and clean up.






The roughed-in cut. Use a small file to clean it up.




To demonstrate the fact that the bolt and extractor don't make
contact with this area. Dummy round is inserted for this photo.





Chapter 2, the larch....the larch.

Installing the rear sight block RSB. Center this over the
centerline mark on new barrel. Compare orientation with old barrel
assmbly by looking at the face of extractor cut-out and how it's
oriented under the back of the RSB. Make sure the new alignment
looks to be the same.

Scribe small alignment marks on the barrel and RSB collar
so that you can maintain position while setting it up for
pressing.





Measure and scribe a line for the RSB's final resting place.
1.880"





Pressing the RearSightBlock. Make sure and use a softer metal
to separate the ram from the barrel surfaces. I'm using a block
of aluminum.




Chap 3.5 Gas Block

Find the placement of the gas-block by installing gas-tube and locking
it in place. Stick GB into tube and hang it outside the barrel to get a
reference scribe mark. This mark is not super critical at this point.



Before pressing on the GB, check visually. Look for parallel lines
across the side with the RSB.




Pressing the gas-block. Press until you reach your scribe
mark. Then remove and install gas tube to insure it will
fit. I had to back the GB off the first time, it was too
tight and would not fit. Don't get it too loose either,
a nice snug fit.




Chap 4 Front Sight Block

Pressing FSB. This takes more acute visual alignment. Sight across
the top and across the sides looking for parallel planes.
Scribe a mark 1.80" back from the face of the threaded barrel end.
This will be the mark to line up the back of the FSB with.
Check alignment after pressing. I screwed on a slant break
after this step just to make sure I didn't have an ugly gap
between it and the FSB after locking into the detent pin.




Chap 4.5 Drilling for Pins
(I sense many builders are nervous about drilling for pins)
forget about it.....

Let's start making chips, .156" bit (shank measurement)
is used on the RSB. Use a new sharp bit for this and
check visual alignment from various angles.
The bit will tend to want to follow the RSB holes because
the barrel is softer. Don't be nervous, this step goes
quickly. I drilled from one side across to the other, it should
zip right across. If it gets hung up, then you must stop
and correct the situation. I've never had a problem with
this step.



Drill for the small pins on the FSB and GB, all 4 are the same.
I used a bit .096" (shank measurment) for this.



I used a nail set for the small pins. They should go in fairly easily
just like they came out.




Install muzzle detent spring and pin first before driving side pin.




Chap 5 Hand Guard Retainer

Time for the layout of lower HG retaining pin notch.
I aligned a finished barrel parallel with the new one and
squared across to get a couple marks.



Use a round file to start the notch. Go slow and don't get
sideways. I used the carbide burr to open it up once I got it
started.




Chap 5.5 Drilling Gas Port

I use a hand held drill. Trust the force Sam, or Fred.
You will need a # 27 bit (.142")
Go slow and easy, keep it in alignment with the GB ports.
Use cutting oil if you want. Make sure not to ram the bit thru
into the opposite side of barrel when it breaks through.




Take a brass bore brush and clean out burrs from drilling.
I had a hangnail in the bore and had to use the blunt, non-sharp end of the
rod to break it loose. Do this very very gently, this is a new
barrel.



Finished barrel assembly ready for final pressing, headspacing and drilling
for barrel pin. Mine will be for an M70AB2 Yugoslavian Underfolder,
semi-battlefield pick up look.



Chapter 6 Barrel pressing

Barrel will press into receiver as normal. I lightly apply anti-seize paste to the
inner surface of trunnion. I use Xebec's press kit. The new barrel pressed in
very nice. I edged it in at the front of trunnion where the crest of the chamber
was almost flush with front face of trunnion.




Remove from press and check the headspacing. Insert a No-go gauge and see what
happens.




I got it right the first time. Showing a strong 1/2" gap of the bolt refusing to move
forward with the No-Go gauge. I next checked it with a Go gauge. The bolt
closed easily with no resistance. Next, I chambered a dummy round and
it went into battery easily. This is the headspacing method I use on the
7.62 x 39 AK's It has proven reliable on dozens of builds.





Chap 6.5 Drilling for the barrel pin

This will be a replay of the previous pin drilling only on a larger scale.
The stock Yugo bbl pin dia. is .275"
I used an 'I' drill bit (.270") for this step.
Don't get excited. Use a fresh new bit, go slow because the tendency for
the bit to overheat is greater here than before.
I did use cutting oil to keep it cool, and I turned it over and drilled from both
sides to belay the tendency of the metal surface to self harden from
drilling. The trunnion steel is very hard stuff and kept the bit on track beautifully.




I poked a Q-tip with a bit of anti-seize down the pin hole and proceeded to
press. The pressing went very smoothly with just the correct amount of tightness.





Chap 7 the finished build


Battlefield pickup look with new barrel. The scope mount I fabbed myself.












You can do it!

[ohmysac]

Very nice tutorial. This will come in handy. THANKS!

[Toten Kopf]

Nice job pardner...

[dutchkma]

Great job. I can hardly wait to see the barrel press and head spacing chapter. I had a positive experience drilling a gas block retainer pin hole in a Chinese barrel, I was amazed how much softer the barrel was than the gas block. This may sway me into one of the new M70 kits.

[Steelcore1964]

this is the best post I have seen in almost a year, I had all but given up on seeing something this good again, How did you get the gas port inside the barrel to come out in the grove instead of the lands?

[ozzy the nuke]

[Rahatlakhoom]

Originally Posted by Steelcore1964

How did you get the gas port inside the barrel to come out in the grove instead of the lands?

I didn't. It fell where it may. I've looked at some of my best shooting AK's.
The port was right into the lands. I don't think it's a critical deal, however,
I'm sure there is a way to do it.

Glad you liked the post, thanks.

[k98k792]

OUTSTANDING!!!!!!!!

[kernelkrink]

Great tutorial! There is a better way of eliminating the burr though.

Old Gunsmith's trick for drilling holes into the bbl without leaving a burr or drilling too far into the opposite side: Measure exactly how far down the bbl the hole will be. Drive a FMJ bullet down the bore to this exact spot, tape wrapped around the brass driving rod will act as a depth guage. A .308 caliber brass jacketed bullet works well, and is easier to drive in than a steel jacketed bullet. Drill into the bbl and when you get brass chips then lead chips, you know you are far enough in. Drive out the bullet and look at your perfect gasport hole.

Ak47 holes do not land in the groove except by chance. Being oblong, it's hard NOT to hit a land. The vertical gasport on an AK74 is located in a groove to reduce jacket stripping with the much higher velocity 5.45 round. Same reason they went to vertical from slanted gasports. As to how they locate it, would probably be easiest to just mark the outside of the bbl when rifling it, have a jig that puts a punch mark when a groove cutter reaches the proper depth in the bbl or lined up with the mandrel when hammer-forging. The rest of the cuts would be made off that GB indexing.

[Z_recto]

Very nice write up and pics.

Now when are you going to do one for the US made barrels?

Waiting for the press and pin installment now.