They do have a twist though, with them the question is what RPM does the pellet have when it leaves the muzzle, which will give you the equivalent of a normal twist rate.
I think it's well accepted that only the last centimeter or less of the bore decides the shot, but the previous however many centimeters do decide how it arrives at that end - for instance if you use a pellet with a too small a head size it will effectively 'rattle' down the bore & it will be a lottery just how it arrives at the end, just a little but enough to widen the groups?
I'd be interested in what other people think on this, & happy to be wrong...