Actually if you took two identical diameter barrel blanks of 26" length , & bored one for .223 & the other for .300 Win  
the .223 barrel will be heavier!

BTW, when I was enrolled in metallurgical engineering, they made sure we could spell things like "metallurgical" and "beryllium."

Don't cite another man's credentials to reinforce your ignorance.

(SCREAMING OUT LOUD)  Removing metal away from the centroid of a bar of a given diameter will reduce its stiffness and resistance to bending and elongation under axial and radial forces.

Didn't daddy take mechanics of materials 101?

Ok.  Let's look at the I-beam example.  Let's say the I beam is cut from a square bar of steel.  The I beam will have the same stiffness as the original beam only at the outer points of the beam.  The center where the material was removed will be less stiff.  If you measure the resistance to flex along all surfaces you're going to find that the original beam had more resistance.

I just received an upper similar to the one described,and they claim 20% weight reduction.
That's one serious hunk of metal hanging out there any weight you can lose will be a welcome addition!
TF