Large-angle convergent beam electron diffraction is used to show that hollow tubes, 5-25 nm in diameter, observed in α-GaN grown on sapphire by metalorganic chemical vapor deposition, contain screw dislocations with Burgers vectors c (0.52 nm). It is shown that these coreless dislocations are not in an equilibrium state but may arise by the trapping of dislocations at pinholes at an early stage of GaN growth. Although pinholes may contain a, c and c + a dislocations, it is argued that only those containing c dislocations can survive to generate nanopipes of constant cross-section along the [0001] axis, as observed experimentally.