One reason that I know of is TC changed it's positon regarding the sale of breechplugs and now refuses to do it, so if you booger it up, you may have an unsolveable, or at best, an expensive repair on your hands trying to find a third party to fix things up for you...and TC breech plugs are a non-standard thread size, with some variance from model to model.
IMHO, you'd probably be much better off buying a TC Hawken "drop in" smoothbore barrel from Green Mountain...I bought one of their .62cal smoothbore flint barrels and it's outstanding