Years ago, in the 70's I bought a Numrich Arms underhammer in the 58. This rifle had ratchet-like rifling. The ordinate bearing side was the high side. I'd always thought this was done on purpose. This barrel was quite accurate and capable of 1 1/2moa. At 100yds. Due I suppose to the 72"twist, it started shooting well at 140gr. 2F.
I have a couple of good closeups of paradox muzzles. One is particularly good at showing.the ratchet rifling.
here they are
1st - slight ratchet:
2nd - Ratchet:
Normal, wide groove, narrow land: