I'm getting an off the wall idea for a wildcat cartridge. How about a NEF single shot chambered in 6.5-.303 Brit? Lop off a .44 Mag NEF barrel just ahead of the pivot lug, bore and thread the remainder for a 6.5 barrel blank, and let Dave Manson work his magic on an "improved" .303 chamber reamer with most of the body taper straightened out, a sharp shoulder angle, maybe 35 degrees, and a neck just long enough to get a good grip on a .264 bullet. The extractor/ejector system ought to work with little, if any modifications needed. Sounds like a fun single shot to me!