Abstract: A strut tool including a hook portion for engaging an upper receiver assembly and a pivoting portion for engaging the takedown pin of a lower receiver assembly of a firearm that acts as a removable strut for holding the upper and lower receiver assemblies in a relatively aligned open angular position providing access to the inner bores and mechanisms of the firearm. The pivoting portion includes a bore accommodating the placement of the takedown pin in conjunction with the lower receiver assembly providing a pivotal connection permitting the angular alignment of the upper and lower receiver assemblies in an open and fixed position. The hook portion includes a takedown block engagement region that provides a means to accommodate the insertion and engagement of the takedown block present on the upper receiver assembly for the purpose of reversibly positioning and holding the strut tool in said relatively aligned position with respect to the lower receiver assembly.