Just as accurate as any other average quality barrel.
Maybe. The barrels aren't the issue. The biggest two accuracy killers with single shots are the hinge and headspace. Any wiggle in the hinge will translate into bigger groups. This is why Encore users shell out the money for oversized or adjustable hinge pins.
The other issue is headspace. A single-shot is different from a bolt action in the way it headspaces. In a bolt action, the base of the cartridge is held tight to the bolt with the claw extractor. The headspace is the distance between the cartridge shoulder and the end of the chamber. Very simple.
In a single-shot, you've got that same shoulder-to-chamber space but you've also got another variable. That being the distance between the primer/cartridge face and the receiver face. That extra distance can vary SIGNIFICANTLY from gun to gun.