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.     
Current Rifle Cartridges
Available for sale
produced by Wilson Combat
using new 5.56 Lake City
brass, H/S 2013 and
loaded with a 125 grain TNT
by Speer FMJ Projectile.
Note; see comparison photos, Left to right
5.56 NATO, 7.62x40WT
300 BlackOut