You can get one through the Accessory barrel program at H&R/NEF. The muzzle loader barrel will fit on either the shotgun receiver, SB1 or the centerfire rifle receiver, SB2. .50 Cal Huntsman blued barrel, $85 + $15 fitting fee and $10 return shipping. See the FAQ at the top of the NEF Centerfire thread. There is a lot of useful information there.
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