IIRC, Spencer repeating rifles used a rimfire cartridge in 1860. Smith and Wesson was making larger caliber metal cartridges by the very late 1860s and Colt and Winchester were producing them by the early 1870s. If your game is in that general time frame or later, you can always fudge history a little one way or the other.

Also, Smith and Wesson was making smaller .22 cartridges as early as the 1850s.

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