6.5mm-300 Weatherby Magnum
History of the 6.5-300 Weatherby Magnum cartridge: Originally in the late 1950's a 6.5-300 Weatherby was designed by Lt. Col. Paul Wright of New
Mexico by necking down the .300 Weatherby Magnum case to accept 6.5mm projectiles and Mr. Alex Hoyer a gunsmith in Pennsylvania built the first rifle. The
Wildcat cartridge was known as the 6.5x300 Weatherby-Wright-Hoyer. (Per P.O. Ackley Book for Shooter & Reloaders Volume 1 & 2, 1966).
Now in 2016, the Weatherby Company claims Roy Weatherby built a 6.5x300 Weatherby Magnum in the 1960s, but never put it in production, so who was first,
I am not sure.
Below from Weatherby web-site:  
“ NEVER HAS A 6.5MM BULLET GONE FASTER! In 2016, Weatherby ushered in a new era in ballistic superiority by introducing both a new cartridge and a
new caliber of bullet to the Weatherby line.
The 6.5-300 Weatherby Magnum is the fastest production 6.5mm there is. Combined with high Ballistic Coefficient (BC) projectiles designed for long range
shooting, the velocity of the 6.5-300 Weatherby Magnum opens a whole new world of possibilities and provides hunters with the terminal performance every
Weatherby shooter is accustomed too.
Weatherby; once again proving that nothing shoots flatter, hits harder, or is more accurate; now in 6.5-300
Weatherby Magnum.” end quote.
3,531 feet per sec, with a 127 grain projectile,  Energy 3,088 pounds per square inch at 100 yards.
Single American rifle cartridges