Geissele offers a new alternative to the barrel clamp or action block for the AR15. This “Reaction Rod” slides into the upper in place of a Bolt Carrier Group, and locks into the barrels locking lugs. The other end has flats that fit right in your vice.
I bought one of these have have used it on a few uppers. I found it easy to use and much prefer it over the action block I was using. Only downside I found is that it doesn’t fit well or deeply in my White Oak barrels locking lugs. Both Colt barrels I tried it with worked much better. Despite not sitting as deep in the WOA barrel, it works very well. I recommend it.