Have you ever heard of bending an Upper receiver?

[cory]

+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.

Klem do you know what material that clamp is? FWIW I'd never use steel to clamp the steel barrel. I'd want a softer metal than the barrel. That way if the barrel (where it's clamped) or the clamp is to be inadvertently damaged in any way, I'd prefer it be the clamp.