Sorry Dave, been out of pocket for a while.
Layne Simpson did some tests on the Light Magnum in Shooting times a couple of years ago, and then talked to Hornady regarding the ammo. Although Hornady proclaimed a 'proprietary' (indicating, but not saying 'duplex charges'

process in building the ammo, Laynes results were very similar to my results when I tested Vihta 500-series powders in 308 and 30-06. In the 308, I tested with 18', 20', 22' and 24'. The chrono figures in the long barrel were far and away the highest by 200fps. That extra few inches gives the powder the time it needs to completely combust. In a short barrel, unburned powder will shoot out behind the bullet.
It wouldn't suprise me if Hornady was using Vihta 5xx. I haven't pulled any bullets to see, but I know enough about ammo manufacturing that duplex load development is labor intensive, dangerous, and time-consuming.
Now the Hornady Heavy Magnum - that's another story!