Is this a 6mm and a necked-down .250/3000 case? Anyhow, a reloading data book would provide comparative case dimensions and capacities. From that you could get a fairly good idea about velocities.
I have found that the 6mm Sierra 85-grain HPBT gives me very tight groups and is totally ruinacious on coyotes and suchlike. Works okay on smaller deer in the dressed-weight 120-pound size, with neck shots or cross-body heart/lung shots. Granted, I've never needed to take a shot past around 150 to 200 yards.
You're from BATFE? Come right in! I use all your fine products!