Have you ever heard of bending an Upper receiver?

[NugginFutz]

Any tool which locks inside the lugs on the barrel extension, transmits no torque through the barrel and with proper lubrication very little will be on the pin. Several things have improved since 1968, this is just one of them.

It makes no difference which end of the barrel you secure. The same torque stresses will be seen at the junction where the pin and slot meet, whether secured in a barrel clamp or with a tool inserted into the barrel extension.