That depends on the game laws where you live. In the US, each state has their own fish and game laws, and they vary from state to state. Check the game laws in your state.
You can go to the tpwd website and do a search. It says that muzzleloaders which are pre-1898 models, or replicas of those models are lawful, but that's it.
As long as the gun doesn't use centerfire...