Jrkrk, I have two of their drop-ins, one in .40 caliber, and one in .50 caliber. Both are excellent buys. I replaced an existing .50 caliber barrel on my T/C Hawken, and the new .50 is accurate as can be. The only drawback I have is the ram rod. It really isn't "period correct". That, however was resolved by making a new one from a hickory dowel.
The .40 caliber was already on a new T/C Hawken stock. It was purchased from the Log Cabin Shop in Lodi, Ohio. The .40 is a tack driver, and uses very little powder. It's great for plinking, or serious target work. It groups under 1" at 50 yards.
Both barrels seem to have a small amount of "choke" near the muzzle. The ball starts a little tight but ends up seating very easily. I also have the same phenomena in two other rifles with G/M barrels, and I've read where this is "natural" in the way the barrels are drilled.
The best buy I've seen on them is from Track of The Wolf. They go for about $140.00, plus shipping. I'm getting ready to buy another one for an older, first year T/C in .45 caliber.
PS - If any of you have a Lyman GPR, Track will handle the necessary modifications so it will fit your rifle, for a slightly higher charge.