You're right on, roundball. I am also disappointed with Green Mountain. I've considered buying their barrels for restoration projects and making over a percussion rifle to a flintlock. I'm seriously reconsidering that idea. There are other barrel makers out there.
Saying that an M6X0.75 metric thread is 'close' to an SAE 1/4X28 thread is simply nonsense. While the diameters are not greatly different, the thread pitch is just nowhere near the same, and the fact that you can get the nipple started doesn't change that. It's wrong. I think they've confused the 1/4x28 coarse thread with 1/4x32 fine thread. Those certainly are not interchangable and neither are M6X0.75 and 1/4x28.
Saying that an M6X0.75 metric thread is 'close' to an SAE 1/4X28 thread is simply nonsense. While the diameters are not greatly different, the thread pitch is just nowhere near the same, and the fact that you can get the nipple started doesn't change that. It's wrong. I think they've confused the 1/4x28 coarse thread with 1/4x32 fine thread. Those certainly are not interchangable and neither are M6X0.75 and 1/4x28.