Assuming we are talking 1866, U.S.A. all the way. The U.S. Army had the experience, in combat, with new technology, and logistics. Whereas the C.S.A. had done well with what it had, the U.S. Army had utilized the most recent developments in military technology as well as logistics in the field. As soon as the war ended, the U.S. govt. began the Allen conversion of Springfield rifled-muskets to breechloaders. Although the Dreyse Needle-Guns were innovative, breech-loaders manned by independent minded combat veterans would have been more than a match for them - veterans that well may have included CSA vets. Muzzle-loading artillery also reached its apex in the ACW. Although breechloaders were coming onto the scene, they had yet to replace the muzzle-loaders.