  Weyl-semimetal superstructures with a spiraling position of a pair of Weyl
nodes of opposite chirality can host a chiral-symmetry preserving Fermi-arc
metal state, where the chirality is carried by cylindrical Fermi surfaces,
electron- and hole-like depending on the chirality. The Fermi surfaces nest at
vanishing momentum separation (zero nesting vector) at the
electron-hole-compensation energy because the nesting is topologically
protected by vanishing spatial overlap of any pair of equal-momentum
opposite-chirality states. In this work we show that the nesting and Coulomb
interaction drive a spontaneous chiral symmetry breaking in such a Fermi arc
metal, which leads to a dynamical axion insulator state but without breaking
translational symmetry (no charge-density-wave order) as in a conventional Weyl
semimetal. As for material realization, we discuss magnetically doped
Bi$_2$Se$_3$, for which the Weyl-node positions depend on the order of the
magnetic dopands. In this case, the axionic condensation can itself stabilize a
spiral order of the magnetization, and hence the spiraling node positions, even
if the magnetic interaction is intrinsically ferromagnetic.
