I dug around the internet a little more.. Seems the 'official' data on the arbor pin talks about a .222" diameter, so 5.64mm then. A quarter inch would be 6.35mm. I've looked at a few more pictures, and I think the ground flat arbor pin isn't going to be of much use. A hollow replacement arbor pin should be easy enough to fabricate though.
The more I think about it, the more I'm sure this is actually a really easy modification. And that's capital E easy; machining the parts needed shouldn't take more than a few minutes, really. Fine tuning the mechanism could take a little longer, but it's still pretty much a lazy Sunday afternoon project..
Here's a quick schematic view of the parts of the modified arbor pin, and how they go together, both in side view as well as top down view. It's pretty much self-explanatory, I suppose. I left out the spring in my drawing, but you know where it goes..
The dimensions are bound to be incorrect - I worked off the assumption a pin diameter of 6mm would be ok, gave it a 1mm wall thickness. The length of the hollow pin is 100mm, 110 if you include the head of the pin.
Ingenious idea of building an internal gas piston into the arbor pin Akumabito.
I understand exactly how you plan for it to work. I have some observations and two questions about it.
I noticed in this concept that you abandoned your earlier shock absorbing dual piston segments that you showed in your earlier first shock absorbing gas cylinder/piston. That will mean this piston doesn't delay to slow down the operation like your other one would.
In looking at the gas tap hole in the arbor pin/gas cylinder, even if you align that hole with the hole in the bottom of the barrel, how are you going to stop gas leakage between the arbor pin cylinder and the barrel at that point?
That juncture where the gas cylinder's hole to tap gas from the hole drilled in the bottom of the barrel, has to be leak free so no gas is lost. The pin has to be removable to take off the cylinder, so you can't weld the pin to the barrel. So how do you propose to seal the area between where the barrel gas hole and the cylinder's gas hole align?
Just aligning them so the bottom hole on the barrel is over the top of the gas cylinder's gas hole would get some gas into the cylinder, but you will have inconsistent pressure from shot to shot and leakage problems at that juncture.
The other question I have is,....the piston being so thin in diameter to be able to fit inside what is now a very small diameter tube inside the arbor pin, would that piston be too thin and delicate to stand up to the stress of operation of it flying back to push against and cock the hammer? It is only going to be about as thick as some firing pins. Unless it was made out of titanium or some really strong material, and even then.....I wonder if it being that small a diameter will take the stress of operation of repeatedly pushing against the hammer without bending and then breaking or jamming from bending?
Like trying to push a weight with a piece of wire.