I came up with an idea to turn a 0.8mm hole in a .6 MIG electrode into a smaller hole...
take one of these:
Actually I used something like this, but same principal:
Stick an M6 thread into a bit of plate to use as a handle on the tip.. and slowly work you way down till it parts.
It ends up looking like the opposite number to this:
Last step is to drill out the length of the body to 3.5mm. laying your 'handle' on the drill press table gets you straight, but then you need to hold the tip from spinning (vice-grips). Then drill as close to the end as you dare.
The resulting nozzle hole is pretty much the same as my 0.5mm purchased nozzle, and it looks like the reduction is .1 to .2 mm deep.
The nozzle works very will in my new extruder with all metal isolator.
More on that later.