I used to do it manually – mount a used gasblock in the mill, find the hole center, swap for the virgin gas block, and drill. I came up with a short threaded rod for the front sight hole so I could establish level,
and a nut threaded for the gas cylinder so that the block would always teturn to the same position in my mill vise.
While this was adequate for the relatively rare virgin block, I also had plans to do a run of L1A1 barrels. Many Australian parts kits came in after Republican AG Gonzales “reinterpretted” the Republican’s
import ban to include barrels and recivers (non-sporting), so the barrels had to be chopped.
I had plans to do a run of US made L1A1 barrels and having 150 virgin L1A1 gas blocks to mount, I had to come up with a faster, repeatable method.
the original pin is grooved – the best commercial replacement pin I could find that would give a tight interference fit is a M2.5x8mm
Here is what we came up with. The constant dimension on all the gas blocks is the gas chamber hole (except Indian, which is a little tight).
A new L1A1 block will have parallel sight wings, but a used one this may not be so. If in doubt, I use a threaded rod in the front sight hole.
L1A1 barrel should have a horizontal flat at 12:00 on the chamber side, while FAL barrels you can use the vertical wrench flat – after verifying they are parallel.
For the first dozen or so, as I refine the locations, it’s a bit annoying. Because I plot one thing (from hole back to zero point) on an existing L1A1 gas block and I get -15.25mm / – 65.53mm.
But then starting at the zero point, I go back to the pre-dimpled point on a virgin block and I get + 15.39 / + 64.78. Then the next one I get + 14.99 / 65.38. So far my max variation is +0.26mm / + 0.75 mm.
Not terrible, but I figure if I log each used and new unit, when I get a dozen data points, I can take the average.