The need to assess the flammability and burn resistance of thicker metal samples tested according to the test methodology of ASTM G124, Standard Test Method for Determining the Combustion Behavior of Metallic Materials in Oxygen-Enriched Atmospheres, and to better understand the flammability of thicker cross-sections used in oxygen applications continues to grow. ASTM G124 describes the typical flammability test sample as a 3.2-mm-diameter rod with a uniform cross-section along its length. Exploratory testing is provided herein to serve as a guide and basis for the future development of a test method to test larger diameters using a tapered sample configuration. The tapered sample allows the attachment of a promoter to a 3.2-mm or 6.4-mm rod section (similar promoter to that used in standard ASTM G124 testing), which then tapers out in a conical shape to enlarge the diameter/thickness of the test sample. This method allows for a practical way to ignite the test sample to evaluate burning that may progress into the larger thickness. The tapered section also provides an indication of where the test sample self-extinguishes (referred to herein as the “extinguishment diameter”). Testing was performed to evaluate the extinguishment diameter of three proprietary alloys and stainless steel for comparison with available straight-rod flammability data and to determine the suitability of this approach as a test method for metals with a larger thickness.