I feel the need to weigh in here.
Sabot--I have to disagree with you about the Rubio Sherman barrel.

In my most humble opinion the CMD barrel is superior.
I think that the price range given is a bit too narrow. You can pay as low as $4 for some Elefant barrels and as much as $30 for some of the incredibly complex Model Point barrels.
Here are the companies that make barrels that I know of and my thoughts if I have actually seen the barrels or used them:
Jordi Rubio - In general they are very good. Sometimes the mantlets provided are a bit rough, but on the whole they are well worth the cost. All major combatants in WWII.
Elefant - Hit or miss. They tend to make barrels to fit a particular kit rather than try for accuracy in my experience. It seems that they just make a turned aluminum copy of the barrel that comes with the kit. A good example of this is the barrel for the Italeri Sherman which was a metal copy of the completely incorrect barrel that came with the kit. Mostly German, but they do have other stuff as well.
Armo - Simply wonderful. Very reasonably priced. Some come with resin and PE components like their SIG 33 barrel which has a resin recoil mechanism and a PE insert for the rifling. Mostly German and Soviet. Get these through Jadar (link at bottom).
Model Point - Incredible, but pricey. Like Rob said they are more like precision instruments, but you pay through the nose for them. Mostly Soviet WWII and Soviet Modern, but some other modern as well.
Barrel Depot - Decent quality, nicely priced. Mostly modern.
Chesapeake Model Designs (CMD) - Very nice and reasonably priced. Not an extensive line, but they are considered by some to have the most accurate line of Sherman barrels and I believe they have a T-72 barrel as well. Buy direct.
Eduard - They only make 5 barrels so far, but they are beautiful. They have Panther G, Firefly, T-72, Tiger I, and Tiger II barrels so far. They are reasonably priced.
Michael Dobiesz - Nicely done and he is willing to make custom barrels (I don't know how much that costs though). Mostly WWII.
New Connection - Nicely done and reasonably priced. Mostly WWII.
CMK Hot Barrels - Very nice and come with resin mantlets. Reasonably priced. Mostly WWII
Fine Molds - Very nice and reasonably priced. Mostly WWII Japanese, but they have some German and Soviet as well. Best source is HobbyLink Japan (link below).
There are some others out there, but I'm pretty certain that these are all of the major barrel manufacturers. I would recommend all, but Elefant with some reservations (only if the kit barrel is correct, but a pain to sand and build).
The best sources for these barrels IMHO are:
Michigan Discount Models Jadar The Barrel Store HobbyLink Japan Panzer Shop Squadron ...and as a last resort
Great Models HTH