Oval bores are exactly that. No grooves, just a bore that's oval in cross section and twists like conventional rifling, imparting spin. One of the claimed advantages was a reduction in fouling. Patented by Lancaster's son, Charles William Lancaster. I don't recall seeing one that wasn't from Lancaster.

I've never shot one. In speaking to people that have, I can't say that I've found a consensus. Some have found them accurate, but many haven't.

I've always wondered about Denys Finch-Hatton's double rifle. Built by Lancaster in 1911 as an oval bore .475, DFH returned it to Lancaster in the late '20s and had it rebarreled to .450 with conventional rifling. Wonder if the barrels were damaged or shot out, or if Finch-Hatton was just unsatisfied with the accuracy of the oval bores.