300 WSM Velocity

[Doug Brownlee]

I Have a 300 WSM I am shooting Berger 185 vdl with 68.0 grains of H4831SC with Magnum primers I am zeroed at 300 with a 3 shot group of 2.375 its 5 inches low at 400 with a 3 shot group of 2.752. can you give me some idea of the velocity? My chronograph is broke parts on order.