Bartlein commented on that video:
”
A lighter weight contour when the barrel gets button rifled (Bartlein Barrels only does single point cut rifling) the stress being put into the blank is much greater. As you turn the contour smaller and smaller in size this can have a negative effect on the bore sizes changing and opening up. When this happens you cannot make the bore size smaller. This is not an issue with single point cut rifling. We don’t see this happening at all. Same can be said about threading the muzzle of the barrel for a muzzle brake or suppressor attachment. On button rifled barrels, threading the muzzle this is a common problem with the bore opening up. The last place you want the bore to open up or go sour on you is right at the muzzle's crown edge. Anything going wrong here has a negative impact on accuracy.
The video link below shows this process happening. The point of the video isn’t to pick on other barrel makers but to suggest to people to make the muzzle threads as large as you can to prevent this from happening. The first barrels where all the bores open up are all button rifled barrels. The last barrels are Bartlein cut rifled barrels. Yes the threads are larger on our barrels but we have made barrels with muzzle threads that measured 5/8” and we turned these down from a blank that started out at 2.280” o.d. and have not scrapped/rejected a single barrel because of it.”
From the bottom of this link
www.bartleinbarrels.com