Every rifle I've ever built has been chambered in a CNC and done with single point tooling. The longest I've ever done was a 300 WBY throated for 240's. I'll say that holding tolerance was difficult in the throat area when the end of the throat was 3+" deep in the hole, , but was as close to nominal as any reamed job I have ever seen. I would not want to try doing a 270, but, a 6br is a slam dunk.
I can't immagine doing it without a CNC lathe. The advantages that outweigh the setup time and "unnecessary" effort as some would say, is that you can make dies that actually fit your cases. And, when you decide you don't like them, you can make more. The chamber itself is not all that different from a reamed chamber. IF you decide to make small changes, you can do that without paying again for a reamer. Is this practical for gunsmith work, no. Would it work in a production environment such as Remington or Winchester where they make more than one gun at a time, certainly.
Difficulties... I'd say, don't waste your time unless you use coolant through tooling and don't waste your time with a commercially produced carbide bar that's coolant through. The coolant holes are WAY too small and won't do squat with the chips you need to move. With my own tooling and nice big coolant holes, the chips from a chamber form as something that looks like a "Chore-Boy" in the tail stock of the lathe. I find that stringing chips that flow down the bore and out are preferred to chips that break up and get jammed around the tool.
An NC Mill to make the toolholder is a nice luxury for making your own indexable tools. I make the pockets with an NC mill, tap and then harden. I prefer steel to carbide for the toolholder itself. I harden mine to 60-62 rc. I avoid straight portions on my tooling to reduce resonance in the tool itself, and hold chatter to a minimum. Most of my tools run without making any audible sound at .007-.010 depth and .0005-.002 feed/rev.
Indexable 80Deg inserts are available that use a #1-72 screw and will bore down to a 6mm throat size if you're careful about the holder design. For my 30cal work I use 120 Deg inserts with a .007 nose r. I find in the barrel steel, I've not had as good of luck with a tool holding up for an entire chamber when using the .002 nose radius, thus I use 7. When you're fussy about the dimensions, loosing .0005 off a tool edge is not acceptable.
The smallest I've ever done is in 17 cal but that was dies only. They were in A2 and L6. Those I did with a Bokum bar. They're too small for indexable tooling.
Where there's a will there's a way. However, for competitive use, given what I've learned in my time at this, I would say that without an NC lathe, this is not worh your time. Under no circumstance should it be considered cheaper to single point than to ream, unless your'e doing hundreds of chambers, and even then, I'd have to study costs a while.
