Look into NEF muzzleloaders. Thay have some good and affordable offerings. They also have combo guns like 12guage/.50 cal. ML, and .243 Win/.50 cal ML.

Or, you can buy their Handirifle (with an SB2 frame) and get additional barrels-- center fire, shotgun, and muzzle loader. Extra barrels range from $42 to $100 each. A starter Handirifle in .223 can be found at Walmart or Gander Mountain for around $180.