I have a TC PA Hunter Carbine barrel (22" long) with 1/66 twist that would fit your barrel channel and stock; it's .50 caliber and flint ignition. This barrel is new and unfired. I bought the rifle new and unfired for the stock and lock. I already had two PA Hunters one is 31" and the other is the 22" carbine.
They are outstanding roundball shooters; I use 80 grains of Swiss or Goex 2f black powder with a .495 Hornday ball and .020 patch. One big hole at 70 yards. I would be willing to trade you even up for your 1/28 twist barrel if it's flint ignition; I would like to try sabots and conicals but don't want to buy a new rifle; just a GM drop-in. I can't find one in flint ignition.
Track of the wolf has some barrels that would drop into your stock listed. GM will be making a 15/16" .54 caliber 1/70 twist 32" long percussion barrel for drop-in on TC stocks this winter and will be available for sale in the spring. The production run is for 50 barrels only; so you must contact GM to get on the list if you want one. Speak to Kerry at GM.
They would consider making a run of the same barrel but in flint ignition if there were enough standing orders that they didn't have to keep a great deal in inventory. The minimum production run is 50 barrels and they need at least 35 standing orders to make a run; I urge anyone interested in a 15/16" .54 flinter 1/70 twist GM drop-in to place their order as soon as possible. Again; contact Kerry at GM to place a standing order.
There is a tread running on The Muzzleloading Forum concerning this under flintlocks.
If you want to trade your barrel for my extra carbine barrel send me a Personal Message on the forum board; just click on my screen name and a drop box comes down.