Without exact measurements one can't say for sure. But and educated guess would be 45-70 either 405 grain, or 500 grain bullet. The military adopted the 45-70 cartridge around 1870, I don't remember the exact date. Civilians quickly picked up on the cartridge, and there were lots of different rifles chambered for that round. The military used that caliber up through the Spanish American War, and I know that our local National Guard was armed with rifles firing that cartridge during WWI, and those bullets are still being loaded and fired today. I just bought a bullet mold to cast these bullets in 405 grain, and will be loading them to shoot in my model 1873 Springfield rifle. At Custer's last stand, his men were armed with carbines shooting this bullet. If you measure it, and find it's .40 cal, then everything I just said doesn't count. Sharps made the 40-70 buffalo rifle, among many other calibers, but the 45-70 was by far the most popular, hence the most common. The army had a Gatling gun chambered for .50 caliber, and I think they also had them chambered in 45-70, but I'm depending on worn out memory. Your bullet looks to me to date after 1900 because of the reeded grease grooves on the bullet. They came along later. The earlier bullets the grooves were smooth on the bottom.