I've been thinking about this all night and a couple of folks are obviously curious about how the upper was secured. I've come to the conclusion that the nut was torqued so tightly that either method I tried would have yielded the same result. I could be wrong, but the nut was essentially frozen to the upper receiver due to friction.
I've got an old upper receiver that I may try some destructive testing on. Does anyone have a shot out, junk barrel laying around that they could donate to the cause?

