Sorry for the misunderstanding. There are a few barel makers out there that will provide prechambered and threaded barrels to customers but I HIGHLY discourage going this way.
Unless you ship your receiver to them and they fit the barrel you order to your receiver after it has been fully accurized, you will not be satisfied with the results.
Lets go back to the barrel ordering issue.
Basically when ordering a barrel from one of the top barrel makers, you simply order a barrel blank which in most cases is a precontoured barrel built to the contour you want.
Say your building a sporter rifle you would select from any contours from a #1 on teh light end up to a #6 on the top end. You should match barrel contour with the weight of the rifle you want to build as well as the round you are chambering. For example, you could build a very accurate light sporter rifle in 25-06 using a #1 or #2 contoured barrel.
If you were building a 30-378 Wby designed to shoot the 240 gr Matchkings you would want at least a #6 contour to control barrel torque and whip.
BAsically you would tell the barrel maker that you want a specific bore diameter, .243", .257", .284", .308", etc...
Then you would order the rifling twist which would be appropriate for the bullet or bullets you want to shoot. Example, if you want to build a big 30 cal magnum wildcat to shoot conventional bullet weights from 165 to 200 gr, the 1-10" twist would be the correct twist. IF you wanted to shoot the 125 to 150 gr class bullets out of your new wildcat, the 1-12" twist would be perfect.
IF you wanted to shoot the 240 to 250 gr ULD bullets, the 1-8" twist would be correct.
Really when ordering a barrel, it does not have anything to do with the chambering you will be using, it is mearly ordering a barrel black of the proper contour, bore diameter and rifle twist for the project you have in mind.
Then the reamer makers and gunsmiths come into play when designing a specific new wildcat. Say you want to design a ne wsuper magnum around an existing factory case for example lets use the 30-378 again.
The best thing to do is to contact a quality reamer manufactourer and have them send you a copy of their reamer print for that specific round. Reamer prints can be very intimidating for someone not used to looking at them.
Here is a reamer print for the 338 Kahn
As you can see there are a few different dimensions. What youwould do it modify the dimensions for your wildcat, say you wanted to reduce the body taper, you would make the change in teh shoulder diameter, say you wanted a tighter neck fit, you would change this dimension.
Once you get the dimension changes made you would submit the modified print back to the reamer maker and they would run the changes through a computer program which will tell you if the changes will work properly for a cartridge. Say your neck diameter is larger then your head diameter, this would not work, extraction would be a problem!!!
The computer will find these problems if they occur.
When the reamer print is OKed to work they will print up a new reamer print for your new wildcat with your name on it and send it back to you for approval and generally for a final signed Ok to start production on the reamer.
Once the reamer is made, it is now time to have the custom dies made up for your new wildcat. I use Redding simply because I feel they make the best conventionally designed loading dies on the market.
For my Allen Magnum rounds, I had to send them copied of my reamer prints and they are able to design the tooling needed for your wildcat off your dies. This can be expensive, up to $300 for tooling alone if they do not have tooling on hand to build the dies to your specs. Also, there will be an engineering and set up charge for the initial dies.
Getting true custom dies built, I mean the one of a kind stuff that has never been done before is expensive for the initial tooling and set up cost. Expect to pay anywhere from $200 to $400 for that first set of dies. After that they are just the standard custom die price.
This is why designing a totally new wildcat can be very expensive, especially if you are building a rifle for your own purposes and not to resale the product.
So now you have the barrel blank, custom finishing reamer and dies on order. Now it is time to send all your componants to your smith and let him build your dream rifle in your new wildcat super mag round!!!
Dies can also be built off from sending a die manufacturer three fired cases. This means that you have to fireform three cases in a completed rifle and send them off to the die maker.
If your wildcat can be formed with other dies for example say you design an improved 30-378 Wby and factory ammo an be fired in the chamber and fireformed that way, this is an easy way to go.
But if you have to dramatically change the dimensions of the parent case you will be better off having the dies made off the reamer prints. THis is much easier for the die maker anyway.
The first step really in designing your wildcat is to design teh reamer. This is the starting point, get teh reamer and then the rest of the steps such as rifle building and die making can easily be done.
Please do not think I am trying to discourage you from designing a wildcat because I am not at all. It is expensive to design a true NEW wildcat but the rewards are well worth it in my opinion, expect gliches there and there because there will be some which is part of the game.
As far as my Allen Mags are concerned, I had Redding build the full length sizing dies and I will be offering Custom in-line bullet seating dies chambered with the same reamer used in the rifle chamber.