Ruger makes single action revolvers in .45 Colt, and I do not believe there is much of a demand for that caliber in a double action revolver. As Sfury mentioned, you can fire a .45 Colt cartridge in a revolver chambered for the .454 Casull so there is even less incentive for Ruger to make a DA revolver chambered for the .45 Colt. When was the last time Ruger made the Redhawk in .45 Colt?