Call them and ask.
If you have access to a chronograph and an extended range to shoot at you can pretty much find out for yourself. Chrony the load in your rifle, at least 10 rounds for a good average velocity. Shoot the load for groups at 100 yard intervals. The further the better. Make a drop chart. Punch in all your known info into JBM and fidget with the BC until the drop numbers match your drop chart numbers. This would give you a good idea of what it is. The more detailed you get the closer your outcome will be..ie, don't let the rounds bake in the sun or a warm chamber, make your drop chart in inches of drop if you haven't made sure the adjustments on the scope are precise, and...the list can go on and on.
I would start at around .350 for BC. It may be higher but if a spbt GameKing is only a little over .4 then the flat based Federal round will be lower.
