Depends on if the available gun powders could support something like the .280 Brit. Remember powder technology improved dramatically throughout the war. Earlier powders were much faster burning and would have had difficulty generating a lot of pressure without a lot of powder volume. Burn consistency would also have been a thing. Many cartridges developed after the war made use of the various IMR powders developed in wartime for autocanon ammunition.Dare I say, FAL in .280 Brit (or even do the AR18 in .280)