It's a "trapdoor" Springfield, one of the first metallic cartridge long arms adopted for US infantry troops. May be a converted rifled musket from the Civil War. I believe the trapdoor was designed by Allyn, to convert said muskets economically to breech loaders. If it is the old pattern, it would be 50/70 caliber, a later one would be 45/70.
Some expert will probably be along soon and give you more exact details.