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Barrel Assembly

  • Barrel Attachment
    • Tolerances
    • Sizing
    • Timing/Index
    • Installation
      • Pressing
      • Heat/Chill
  • Rear Sight Block
  • Handguard Retainer
  • Gas Port & Gas Block
  • Front Sight Block
    • Muzzle Brake
  • Gas Tube & Upper Handguard
  • Chamber Casting
  • Reaming

Barrel Components are installed after the trunion is riveted to the receiver.  This section discusses assembly of a press-fit barrel.  The press-fit is the standard assembly method for stamped-receiver rifles.  Some milled-receiver rifles use the press-fit method but most milled receivers use a threaded barrel assembly.

 

Barrel Attachment

So you have that nice shiny trunion riveted to the receiver.  Now what?  The barrel needs to be press-fit into the trunion.

Tolerances

The barrel is press-fit with .003-.004" interference at the barrel journal (the area that is precision-milled for insertion into the trunion).  

There are two common sizes for barrel journals - 23mm and 19mm.  23mm is the 'common' Euro-model sizing.  19mm is unique to Chinese barrel assemblies.  Unless you bought a Chinese kit, you almost certainly have a 23mm barrel. 

Sizing

The best method is to use a barrel that is correctly sized at the journal.  Sometimes this is not feasible, if for example you are trying to assemble a Krinkov kit using a pre-ban Chinese rifle, to allow for the folding stock.  In these cases, you may need to resize the barrel journal.

A 23mm trunion requires a 23mm barrel.  A 19mm trunion requires a 19mm barrel.  This sounds obvious, but wait...

A 23mm barrel CAN BE USED on a 19mm trunion!  This will require using a metal-working lathe to cut 2mm off of the barrel radius (measuring the RADIUS and not the DIAMETER) to resize the barrel journal down to 19mm. 

A threaded barrel CAN BE USED on a press-fit trunion!  The same process applies here - use a metal-working lathe to remove the threads so that only a smooth journal segment remains.  Measure.  If it is less than 23mm then you'll have to find a 19mm trunion for your project.  Note that Global Trades sells 19mm trunions.

A 19mm barrel CAN BE USED on a 23mm trunion!  This requires machining of a sleeved insert that is press fit onto the 19mm barrel.  The sleeved insert is 19mm on the inside, and 23mm on the outside.  This barrel sleeve may need to be permanently fastened to the barrel, by either spot welding or pinning in place.  One could make a tighter-fit sleeve (.005-.006" for example) and use a torch to heat it - thereby allow the metal to expand to a larger diameter.  The barrel sleeve would be nearly impossible to remove when cool, due to the extremely tight press-fit.  This heating "trick" is how flywheel gear rings are sometimes installed in the automotive industry.  Once the sleeve is installed, use the 19mm barrel as you would a normal 23mm barrel.

 

Timing/Index

Technically the AK has no "timing" as for example one would find on a FAL.  If the assembly uses a completely brand-new barrel, then the barrel is pressed and the gas port is drilled into the barrel at a later time.  The gas port is already drilled on de-milled parts kits and therefore there must be an alignment of the gas port to top-dead-center (TDC) to ensure proper alignment of the gas tube and piston.  This is also referred to as the Index.

Simple measurement of the gas port can be made to ensure TDC alignment.  It is easier to align the (already-installed) rear sight block (RSB) with the trunion.  Trunion-to-RSB alignment is usually all that needs to be set.  If the gas port is too far out of timing then the gas piston will bind in the gas tube and/or the gas tube may not be able to be removed. 

 

Installation

There are two common "garage" installation methods - pressing and heat/chill.

Pressing

Pressing requires use of some sort of press or alignment jig to keep the barrel aligned while pressing.  Most people choose to use a 12-ton or greater arbor/bearing press, running in the $100-$200 range, like this one:

1patpress1.jpg (34464 bytes)  

Alternatives to a bearing press can be used, as demonstrated here with a 36-inch I-Bar clamp, running for about $15-$30: 

DSCN3182.JPG (159576 bytes)

Note that some sort of support is required on the bottom of the trunion, so that the receiver isn't damaged from the crushing effect of the press.

Use anti-seize paste (available at automotive stores) to allow for smooth insertion of the barrel.  Depending on the condition of the trunion, touching the inner diameter of the trunion bearing with a brake hone may be needed.

The pressing should be done slowly and deliberately.  Begin slowly.  Try to "feel" the barrel into alignment as pressure is applied.  The tension between barrel and trunion for a matching parts kit should be relatively small.  On my test "kit" the barrel installed with barely any effort.

USE CAUTION!  Note that you CAN insert a mis-aligned barrel into the trunion for the first several millimeters!  This is NOT a good situation, as the tension on the crooked barrel could cause the barrel to "snap" out of the press like a spring and fly across your garage.  If your body is in the barrel's path, you could most certainly break bones!  BE CAREFUL.  Ideally you can protect yourself with some sort of pad or curtain the barrel can bounce into "just in case".

If you use an I-bar clamp the danger still exists, but since you are applying hand pressure to the worm screw, you can better feel the tension on the barrel.  

More information can be found on the Pressing Page

 

Heat/Chill

For those who have no I-Bar clamp or bearing press available to them, a quick-and-dirty method of barrel installation is to freeze the barrel and heat the trunion.  This method is recommended only for matching parts kits, since headspace adjustments using this method could be nearly impossible.  Of course, the condition of your parts may vary and you may have enough tolerance to insert the barrel with very little force at all. 

A frozen barrel shrinks slightly, making the tolerance less than what it is at room temperature.  Prepare the barrel beforehand by secure a penny or some other soft metal to the muzzle of the barrel so that you can have something other than the barrel to hammer on in the next steps.

When the barrel is thoroughly frozen, clamp the receiver/trunion assembly in a vise and use a propane torch to heat the trunion.  Heating it will cause the trunion to expand slightly, increasing the inner diameter.  Put on a good pair of gloves to be ready for the next step.  

When the trunion is adequately heated, quickly remove the barrel from the freezer and quickly align to the trunion.  Next, GENTLY tap on the muzzle end with either a wood block (or the taped-on penny) until the barrel is set in place.  If you go too slow, the barrel will "thaw" while the trunion "cools" and the barrel could be stuck in a half-installed position.  Then you are left with no choice but to drive it out and start over.  

Most people that have done the heat & chill method have reported great success and were surprised at the ease in which it went together!

 

Rear Sight Block

The RSB is press-fit onto the rear of the barrel.  Once fit onto the barrel, a hole is reamed for a drift pin to sit in an interference fit across the barrel, exactly like the barrel pin.

 

Handguard Retainer

The handguard retainer simply slips onto the barrel.  It should be installed before the gas block and FSB. 


xebecBarrelBlock1.jpg (13842 bytes)    xebecBarrelBlock2.jpg (12099 bytes)

 

Chamber Casting

Sometimes you need to make a chamber casting to verify the condition of the throat, and excessive binding issues that may be causing misfeeds.  Making a casting is relatively easy, however care should be taken because the fumes created when melting the casting material aren't good to breathe.  Also, heat will be used so dress to protect your vital areas.

I have used common sulfur and the cerro-safe chamber casting alloy.  I would use either way if I needed to do so again.  The sulfur is extremely cheap, but is stinky.  Cerro-safe is more expensive and requires the use of a dedicated pot or pan.

With either method, you first make sure the chamber is clean and relatively dry of cleaning fluids.  Some suggest swabbing the chamber with a cleaning patch dipped in a light coat of WD-40 before starting to help the casting come out more easily.

You plug the barrel with a paper towel deep enough that the start of the rifling will be part of the casting.  That provides you with a casting of the throat area. 

Secure the barrel in a vise, breech end up.  Chambered actions may require creative use of a spout formed from tinfoil or a metal tube to reach the chamber.

Put on your respirator and heat the casting material.  Be sure to have a fire extinguisher handy 'just in case'.  Also, be sure there is plenty of ventilation - melting sulfur will create a large amount of very stinky fumes!  Be sure not to over-heat the sulfur or it will scorch and start to smoke. 

When the casting material is molten, carefully pour it into the chamber so that it begins to pour out.

Let it cool, and use a cleaning rod to gently push the casting out of the breech.  You now have a chamber casting that you can use to measure throat erosion, gauge the overall quality of the chamber, or troubleshoot issues.  You should label the casting with a sharpie marker so that you can quickly identify it at a later time.

 

Mini-lathe setup for profiling

 

Chamber Reaming

I saved a reprint/reformat copy of the "How I make chamber reamers" web page and uploaded it to my Skydrive site.