I looked into it during my 6.8 build.
I first noted all it would do is center up the adjustments needed to zero. That's ALL it can do. It does not make the barrel more accurate - only the rifling, chamber, and load do that. All it does is make the nose more flat to match the barrel extension ring.
That means the ring was machined correctly in the first place, which is likely due to it being done on a lathe. And then, when it's screwed onto the end of the barrel that the threads match concentrically, which they do - mostly. We are still talking +/- .015". So we mate a mostly coaxial barrel with mostly perpendicular ring to the front of a mostly flat nose and clamp it down with a barrel nut. Hope they got the upper mostly coaxial. As I said, it's all =/- .015".
Second I noted most of the 6.8 builders who offered to square the nose did so EXTRA COST. They were not assembling their MOA guaranteed rifles by doing the work themselves up front. They just stuck in the barrel and tightened it down. And they still warranty the MOA.
All it does is center up the adjustments to the mostly coaxial bore line - =/- .015".
Want to ponder something? Nobody shoots the barrel to group it before they tap the gas hole and grind in the feed ramps. It would be nice to have one that was oriented to 12:00 when assembled. That service hasn't been revealed to me when I search for it. You get a barrel with whatever group it lands on - 3, 7 11, whatever. and then you still have to crank in adjustments to correct. Wouldn't it be easier to simply dial in whatever elevation correction it needed. Not happening. Yet.
So for the expense of buying and using the tool, I got a 6.8 that I have no idea is better centered up or not, and I get to square the nose on AR's when it seems the torque wrench is going over 80 foot pounds but I haven't got the teeth aligned for the gas tube. In that regard the tool did pay it's way.
I bought the tool because I got it for "free" using a military discount on the order. Oh Well. YMMV.