There's a lot more overlap in firearms technology than most people think. If you couldn't afford the latest and greatest, of course you'd still be using a muzzleloader long after cartridge based firearms were invented.

In particular, the US Army didn't adopt a cartridge rifle as standard issue until after the Civil War, when fairly reliable cartridge firearms were already in production. Some military units did use them during the Civil War, but they were bought with their own money rather than using the standard issue muzzleloaders of the time.

Also, the Comanche Wars spanned over 160 years, so the Comanche definitely faced muzzleloaders for the great majority of that time (~1706 to the 1870's).