Please Note: In the picture, left to right 5.56 NATO, 7.62x40WT, 300 Blackout for  size comparison.

One Cartridge, not a box: 7.62x40WT or 7.62x40mm Wilson Tactical by Wilson Combat of Idaho, USA, factory loaded using new 5.56 Lake City brass, H/S 2013 and a 125 grain TNT by Speer FMJ (Full Metal Jacket) Projectile.
History of the 7.62x40WT Cartridge:

This cartridge was designed by Bill Wilson of Wilson Combat and marketed in or around 2011. The 7.62x40 WT, WT for Wilson Tactical, is a .223 Remington or 5.56 NATO case neck out to .308" or 30 caliber and shortened to 40mm so the cartridge will fit in an AR-15  (.223 Remington or 5.56 NATO) magazine.

It is is about 5mm longer then the 300 Blackout and 300 Whisper cartridges.