The staff at 4D tells me this all grew out of the Savage pre-fit barrels they are selling. They custom chamber these barrels for clients in any caliber that they have tools for, and boy do they have tools, well over 600 chamber reamers on hand. Fred said, “We have no desire to hold an FFL so we will not accept actions in the shop, only barrels. We can rechamber them to any cartridge that is appropriate for the barrel in question. Of course we respect the pressure limits of various actions and will not put a cartridge in a barrel that cannot handle it. Accuracy is normally as good as it was originally and in many cases it’s better because we are careful about proper set-ups and tool alignment.”
Things to consider when thinking about rechambering a single shot barrel.
1. The bore diameter must be the same for the new cartridge as it was with the original, unless you are willing to rebore the barrel as well. (Reboring is cheap).
2. You must pick a chamber that is larger than the original chamber, ideally, larger in diameter and at least a little longer. Sometimes the original neck diameter was on the large size in the factory chamber and when you rechamber it leaves a small groove or mark in the neck area. Normally this is not a problem for function, but it leaves marks on your brass.
3. Ackley Improved cartridges do now work well for single shot rechambers unless your starting from a much smaller case, like a 22 Hornet to a 22-250 AI. This is because of how they are headspaced.
4. If you working with rimmed cases then an Ackley design is fine in a single shot simply because the rim handles headspace.
5. The twist rate of the original barrel should be compatible with the bullet you will likely be shooting in the new chamber.
For ideas on rechambering options see this page at 4D that offers suggestions.