You can get .300/221 Fireball barrels at Olympic Arms. I have one of their SUM barrels in 6PPC and it is a serious tackdriver.
Don't discount the .300 Fireball barrels at Model 1... I just got one earlier this week and, yes, it is a plain old hbar but so far it's performed very well. I'm a firm believer in breaking barrels in properly. After I did the first series of shoot once/clean (10 times) I shot a .8 MOA group. Pretty cool.
Scot