A structural model for a 3000-atom Ar cluster has been developed whose simulated electron-diffraction pattern is in excellent agreement with hitherto unexplained experimental results [J. Farges et al., J. Chem. Phys. 59, 3454 (1973)]. Although the cluster size is intermediate with respect to the icosahedral and fcc crystalline regimes, the model does not involve a structural transition. It consists of a persisting multiply twinned core with dominant fivefold symmetry, surrounded by a faulted shell with defects that stimulate 3D fcc single crystal growth and prevent hcp stacking.