Answer:
[image: 5102372_06_1898_springfield_30_40_krag_ca_640.jpg]
30-40 Krag Carbine.
Adopted by the U.S. Army in 1893, Krag was the first small-bore, smokeless centerfire fielded by the army. Roosevelt armed the Rough Riders with it, while some U.S. troops were still carrying 45-70's.
Interestingly enough, Roosevelt didn't carry a Krag, but carried an 1895 Winchester chambered in 30-40 Krag during the Spanish-American War.