Have you ever heard of bending an Upper receiver?

[montana]

The Department Of The Army Technical Manual TM 9-1005-249-34 copy 1968 instructs using a aluminum barrel vice jaws clamp similar to stevegun1 posted on this thread. I have used both the clam shell and barrel vice methods with out any issues. The barrel extension tool like the Brownells or the Geissle seems to be very similar to the barrel clamp method as far as stress concerns to the upper receiver. I use an upper insert to mitigate any stress related warpage when using a clam shell, "my personal favorite method". Using the receiver inserts with pins to install a barrel as LRRP52 has stated in this post should never be used for installing a barrel as they were not designed for this application.