November 25, 2011
George Spafford is building a replica of the Yugo M70B rifle using a stub parts kit. A new installment will appear here every Friday
Install the Barrel — Part 1
Here, an AK-Builder barrel press jig holds the trunnion in proper alignment as Spafford uses his 20-ton press to push the barrel back into place. It's critical to have the barrel straight, not only to avoid canted sights but also any potential binding problems that could cause the barrel to bend. Take your time, and if it feels like too much pressure is being required, stop and carefully inspect everything in case something shifted and is misaligned.
Next up is the reinsertion of the barrel. I put the barrel in the freezer for a few hours so it can shrink. When ready to install it, I insert the receiver and trunnion assembly in a vise and first polish the inside of the trunnion with emery cloth to remove any burrs. You want to remove as little material as possible. I then warm the trunnion with a heat gun so it will expand and apply a light coat of Permatex non-seize coating to help the parts slide together.
At this point, I get the barrel from the freezer, run out to my shop, align the barrel to the front trunnion by sighting down the "ears" of the trunnion and lining up the rear sight block (RSB) on the barrel, and then press the parts together by hand. I quickly double-check the alignment and give a few whacks with a mallet on the protected muzzle end to seat the barrel further.
To be clear, I use a beat up old muzzle brake that I cut flat on the end. Some guys have very good luck using a old blank firing adapter for the same purpose. Regardless of the method, you want to protect the muzzle from damage.
After this initial start, I take the receiver and barrel assembly to the press to finish the job using an AK-Builder barrel jig. There are a number of other proven methods including using a mallet for the whole job, air riveters, threaded rod and other press jigs.
As you get close to the original pin location, take it nice and slow. Clean out the channel with a Q-tip or pipe cleaner so you can clearly see how the parts are coming together. Trust me, it is much easier to press the barrel in than to try and press it back out — especially if you don't have the tools to do so.
Take your time and make sure the barrel channel lines up. This will be the starting point for checking headspacing. With a worn M70, you probably will need to adjust the headspace and use an oversize pin — George did and that will be discussed in the next installment.