Hello Marc,
I can't tell you much about the 20mm vulcan round other than that I know it exists and there are some rifles out there (in the US anyway) chambered for that round. I've seen a Lahti anti-tank rifle converted to that cal as well. I owned a Lahti for some time in the original 20mm X 138 B caliber and I currently own a Solothurn S18-1000 anti-tank rifle in that cal. I do load for the Solothurn and shoot it. It is quite pleasant to shoot and very accurate as well. What else could be expected for a rifle from City/Canton Solothurn???
I don't know if I can help you at all but I'll try.
Frank