This study addresses the design of DOT-4BA refillable cylinders using both experimental and numerical approaches. Using traditional design methods, these cylinders often experience buckling on the bottom end closure during pressure testing. A finite element analysis (FEA) design tool was developed using axisymmetric material nonlinear conditions to predict the buckling of the cylinder bottom end closures. The FEA model was also used to evaluate the influence of variations in end-closure geometry, material thickness, and strength on buckling. In addition, an optimization algorithm was employed to minimize end-closure material (weight) without buckling when they are subjected to their specified test pressure. Experimental studies were conducted via hydrostatic pressure tests with water at the R&D laboratories of a cylinder manufacturer The axisymmetric nonlinear FEA models were developed successfully, and the obtained results are compared with experimental test results from cylinder manufacturer case studies.