AFAIK, there are none readily available for over-the-counter purchase.
There are, however, at least two options.
You can obtain a T/C .56SB SmoothBore barrel w/breechplug (around a 28ga I think) - but be aware that it's 1" across the flats, and IIRC a .50 cal T/C Hawken barrel is 15/16".
Another option would be to use the services of one of the several muzzloader gunsmiths who bore out rifles to smooth bores ( about $70 IIRC), using either your existing barrel or an inexpensive donor picked up somewhere.
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