Have you ever heard of bending an Upper receiver?

[Ullr]

About the same as the Geissele reaction rod but a lot cheaper. Clamp the armors tool in a good vise, insert this in the lugs and there is no stress on the receiver at all.

That is not true. You can't torque the receiver to the barrel nut without applying torque to the receiver and the barrel nut. The same stress is applied to the receiver, it is just all being applied at the barrel pin notch (unless you're also gluing the barrel into the receiver, in which case some of the torque is transferred to the receiver via that glue joint). Why focusing the receiver torque on that notch instead of spreading it out over a larger area of the receiver is considered preferable is unclear to me.

The reaction rod type tools unstress the receiver when working on muzzle devices by isolating the torque to the barrel extension/barrel interface. The torque is then borne by friction on the threads and shear stress on the barrel pin. Far better to clamp the barrel in that circumstance, which isolates the torque to the barrel/clamp interface and removes the possibility of shearing the pin and unscrewing the barrel from the extension.