Saw it in the ranks of BR shooting, was wondering what it is and what the ballistics are like.

I believe it's the 340 Weatherby necked down to .308

It's the full length improved H&H belted magnum in 30 caliber. Very similiar to the 300 Weatherby, but with minimum body taper and a standard shoulder instead of the rounded Weatherby shoulder. Spec's are given in 9th edition of "Cartridges of the World".