They do indeed - the Python bore is tapered at the very end. However:
1. A large part of accuracy is caused by how well the bullet fits the chamber-throat as it exits before entering the forcing-cone - many otherwise accurate revolvers have been ruined by a variance in the throat-diameter between the separate chambers. I've measured mine and they're fortunately all very consistent - this made choosing a size much simpler.
2. The taper only occurs at the end of the barrel. I don't want the bullet undersized while traveling down a .357" bore (for obvious reasons) - and the design of the TL fins (especially with my softer alloy) will allow the bullet to swage down to .355" muzzle-diameter with no fuss, much like a Brenneke slug's fins collapsing when exiting a full-choked shotgun.
It's a juggling act, but 1) I've had no pressure signs, 2) no leading at all and 3) the beast seems to be more accurate than I can shoot her, so no complaints here regarding accuracy :)