The short barrelled rifles are fine for hunting.
You get some velocity loss, going from a 28 inch barrel to a 21 inch barrel, but not that much. Still very lethal with the short barrel.
As has been said, you will have the fast twist with the short barrel.
The 1:48 twist is very effective. I have killed 7 deer and 6 wild hogs with my TC Hawken, using patched round ball. Not as accurate as a 1:60 twist but plenty accurate for deer hunting at 100 yards.
There are replacement barrels available. Green Mountain makes barrels for the TC Hawken, and others, you just drop it in. Takes about 30 seconds to replace the barrel.
You could find a used TC Hawken that somebody didn't clean right. The barrel is ruined but the rest of the gun is fine.
You could pick one of those up for $100. Then you could buy a Green Mountain barrel.
They you have a real accurate TC Hawken with a brand new barrel for around $300.
Here are some Green Mountain barrels for $220.
http://www.brownells.com/.aspx/pid=9224/Product/MUZZLELOADER_INTERCHANGEABLE_BARREL_SYSTEMThe GM TC Hawken round ball barrel is 1:70 twist, so very accurate for round ball, it is 32 inch.
The sabot/slug barrel is 1:28 twist and is 28 inches long.
So pick your poison, if you can decide which projectile you want you will be more accurate with that particular barrel.