DON'T do it! The older Optima and Wolf rifles were steel, but not heat treated to withstand the pressures of the centerfire's open chamber (in other words, the pressures are held in the muzzeloader barrel by the breechplug, the centerfire is not). Newer Optima and Wolf rifles are a high-strength aluminum, but still nothing you should put a cartridge into. And, there is always the legal ramifications of creating a firearm from a "non-firearm".