Cris E. Murray helped design the 6.8mm SPC cartridge for SOCOM and considers it to be the best cartridge for the AR-15 platform and similarly sized weapons. The problem with the 6.8mm SPC, and similar cartridges like the 6.5mm Grendel, is that their performance has been constrained by the necessity of the cartridge to be able to be chambered in a M16 or M4.
In his spare time Chis has been developing the ultimate military cartridge which he calls the 7x46mm Universal Intermediate Assault Cartridge. The design is based on the 7.62x45mm Czech cartridge.
The 7x46mm is designed to replace both the 5.56mm and 7.62mm NATO cartridges. It is low recoil and so can be used from carbines, but has long enough range to be used in machine guns and marksmen rifles. Its overall length has been optimized so that guns chambering it would be bigger than an AR-15 but smaller than a AR-10.
The below photos show how it compares with the 6.8mm SPC. Note: the 6.8mm round was fired from a 12″ barrel, while the 7x46mm was fired from a 16″ barrel.
The below ballistic graphs are for a 7x46mm UIAC loaded with a 130 gr bullet traveling at 2650 fps.
Murray has stated that the 7x46mm is designed to …
used a 130 gr. FMJ projectile having a length of 28.5mm (1.1220 in.) and a BC of .411 or better.
have a maximum cartridge over-all-length (AOL) 64.262mm (2.530 in.).
be used with 7mm Mauser bore diameters of 6.98mm bore, 7.24mm groove, having 4 grooves at a 1:297mm (1:11 in.) twist rate.
be used in 7mm American bore diameters of 7.04mm bore, 7.21mm groove provided proper reamer bushings are used.
to have the same pressure specifications as 7.61x51mm NATO/EPVAT.
This cartridge seems to be more a reference design than a practical cartridge. If the military do not want to switch to another 5.56mm sized cartridge they are defiantly not going to switch to a completely new cartridge. This cartridge would require new carbines, rifles and machine guns to be developed.
[ Many thanks to Hugh for emailing me information about this cartridge. ]