Next generation defense applications for structural steels require new alloys that eliminate or reduce critical materials from their composition, while maintaining or exceeding the high strength and toughness requirements for extreme shock or dynamic loading environments. A new alloy system has been developed for this purpose and characteristics of its manufacture and material response in component manufacturing must be understood. In the present study, the relative machinability of a new structural steel, denoted as AF9628, was evaluated relative to a surrogate steel benchmark (AISI4340). The effects of machining parameters employed in milling across a range of machining speeds, feed rates and cooling conditions was investigated by measurements of force signatures and surface roughness. Machined chips were collected and measured to determine the relative effects of these parameters on evolved microstructure. Changes in chip hardness and microstructure were observed due to the effect of cooling conditions in the milling process. Subsurface microstructure was measured using electron backscatter diffraction. The utility of these methods for understanding machinability for defense alloys and components is briefly discussed.