.264 in (6.7 mm)
The 6.5-284 Norma originated as a wildcat cartridge based on the .284 Winchester cartridge necked down to 6.5 mm.
6.5-284 Norma Wikipedia
The parent case .284 Winchester was created around 1963, but did not see extensive commercial use. The 6.5 caliber allowed for the use of long, aerodynamic bullets. In 1999, Norma submitted it to CIP. It has since been standardized as the 6.5mm-284 Norma.
It has been used extensively in benchrest competitions and is known as an extremely accurate long range round. Using an improved version of the 6.5/284, Rich DeSimone set a 1,000-yard (914.4 m) world record at 1.564-inch (39.73 mm). Rich DeSimone's 1,000-yard (914.4 m) record has been broken by Tom Sarver, who shot a 1.403-inch (35.64 mm) group in 2007 using a .300 Hulk wildcat cartridge that is based on the .338 Lapua Magnum cartridge. In contrast, most hunting rifles do the same at 100 yards (91.4 m) to 200 yards (182.9 m). A 140 gr (9.1 g) class bullet can be driven safely to 3,000 ft/s (914 m/s). Also, there are many popular hunting bullets for this caliber, making it a good hunting round as well.