By varying the parameters controlling the growth of crystals, including the thermodynamic variables, such as temperature, pressure, and reagent composition and the kinetic factors, namely reaction time and cooling rate, we found the most appropriate conditions for the reproducible growth of the nonstoichiometric antiperovskite superconductors MgC1-xNi3-y Bulk single crystals of MgC1-xNi3-y were grown by a self-flux method at 3 GPa and 1700 degrees C using a mixture of Mg, C, and Ni powders in a molar ratio 1:1.25:3. The as-grown black colored crystals, mechanically extracted from solidified lump, exhibit various irregular three dimensional shapes, with flat surfaces and maximum dimensions up to similar to (1-1.2) x (0.8-1.0) x (0.4-0.6) mm(3). Single-crystal X-ray diffraction refinement confirmed the high structural perfection of the grown crystals [Space group Pm-3m, No 221, Z = 1, a = 3.7913(1) angstrom, and V = 54.5(1) angstrom(3)], but also the presence of deficiencies on the C and Ni sites. Temperature-dependent magnetization measurements showed a single-phase behaviour with a critical temperature (T-c) ranging between 6.3 and 6.8 K due to the slightly different C and Ni stoichiometries of MgC1-xNi3-y crystals. The growth of relatively large crystals reported here could provide a helpful guidance for further syntheses of various 3d-based antiperovskite intermetallics under high pressure.