I am concerned about the TC's - a distant secondly, due to their appearance. My primary problem with any factory rifle, is what was used in it for powder. Pyrodex and T-7, along with the new IRM White Hots all contain Potassium Percholates - freely admitted in the 2009 Hodgdon Anual Manual. This stuff in itself is highly corrosive. I've seen 2 TC barrels used for 2 years with the stuff, rotted one end to the other, owned by the same guy who roted two Gm barrels in the same length of time using the same powders. I would be mightily concerned over the condition of any used production rifle's condition inside. Re-barreling is going to increase the price of those to well over what the Lyman costs, which has a brand new Gm barrel, known for their accuracy. Those guns as well, most likely have a buttoned barrel with the normal .004" rifling in their 48" of twist.
A Lyman GPR in .50 or .54 would be much preferred - to me.
I guess that shows where my vote goes, Martin. The Lyman kits are easy to put together, as is the one 'change' to design I'd make, and that's the take the slight hump out of the tang, just bck from the breech .