Have you ever heard of bending an Upper receiver?

[Klem]

There are different methods and tools for barrel work... LRRP52 is correct, you should always clamp your barrel when working on the muzzle.

+1. Always use the barrel to absorb torque when working on the barrel nut and the muzzle device.

In this case Waveslayer's also talking about removing a stiff barrel so it's not just about rotational torque.

For removing stiff barrels I clamp the barrel with a Davidson barrel vise and remove the barrel nut and muzzle device that way. Then clamshell the upper with the barrel and Davidson still attached in a bench vise. Then gently tap the floating 'wings' of the vise with a small mallet, alternating either side as it slowly comes out. May take a while depending on how stuck it is but they all eventually start to move. Just don't get impatient and start pounding it, or lever the barrel to get it out.