The Ruger is the gun that was taken by Jake Chambers from his father, Elmer Chambers. The gun was originally placed in Elmer's study, but was removed by Jake as he was preparing for his return to Mid-World.
The Ruger is able to travel safely between worlds, as are Roland's guns. However, the Ruger, being from Jake's world, is not as easily enchanted as the others. When Roland draws on Marten Broadcloak with his own gun at the end of Wizard and Glass, Marten only smiles and teases him, knowing the gun will not fire against him. But when Roland tries to shoot him with the Ruger, Marten squeals and runs, because he knows that the danger from the Ruger is real.