Hello Pinotguy,

I think you can use all .284" dia bullets and therefore also a standard .284" dia barrel (use 260mm twist).

The german book of GEVA (aproxiamtely what british call a "proof house") gives the bullet dia as .284" and also the reloading data (we already exchanged a mail) is given for this bullet diameter.
Data for (US)bullets range from 120grs/140grs/160grs Barnes X, Nosler 120grs/140grs Solid Base, 140grs/160grs/175grs Swift A-Frame, 160grs Sierra SP175grs Nosler Partition, 175grs Speer Grand Slam.

To summarize:
yes, I think that there are enough US-bullets in .284" and data you can use!

By incident, I was today on the range and shot with my Mauser 66S in 7x66SE vom Hofe the Hornady 154grs Interbond, but precision was not on par with the Norma 156grs Oryx I shot as comparism.
The Norma was together with the Sierra 160grs HPBT Gameking best in precision (ca. 25-30mm for 3 shots).

best regards
Rolf