bkb0000 is dead on, especially about the part of not using bbl blocks for bbl nuts. This puts a hell of a stress on the alignment pin and slot in the upper receiver. At least that's how I see it.
Here is my preferred method. Complete with VLTOR MUR upper in the pic as well.
I do NOT SOLELY use this upper receiver block however. I use it IN CONJUNCTION WITH a wood block on the TOP of the upper receiver.
IOW's, rotate the pictured assembly 90 degrees, and throw a wood block in between the vise jaw and the flat top of the upper receiver.
Voila... Strong, simple, and safe. (doesn't mar up the finish on your refeiver like that stupid Brownells clamping block either). Best method out there IMO....
http://www.rainierarms.com/?page=sho...product_id=510

