Just did a tiny bit of research on the leade in a 22lr chamber/barrel. As you suggested, not much leade but I didn't find a description either.
The remark made said that the rifle shoots better if at least the front driving band of the bullet engraves the rifling when chambered. That doesn't leave much room in front of the chamber for anything but rifling.
It sounds like that tapered cone isn't very long or it wouldn't mark the driving band when chambering.
Some number I have seen describing the chambers suggest a 30 degree taper from the chamber to the bore. That is pretty steep and would definitely engage the front driving band on most bullets.
The resulting groups of your testing suggest that the more the bullet is engaged in the rifling when chambered, the more accurate it might be, at least in your rifle.
The longer the front drive band, the more the bullet is engaging the rifling, the better the group. Possibly due to better alignment with the bore.
I know my CZ 457 MTR is supposed to have a match chamber. When I close the bolt on a cartridge, I can feel the bullet engage the rifling during the last bit of bolt travel.