I'm thinking something is off with either your math or your 243AI.
Per
http://www.6mmbr.com/243win.html ,"The .243 AI delivers more velocity by virtue of enhanced case capacity--roughly five grains more H20 capacity than a standard .243 Winchester. The .243 AI has a water capacity of approximately 57 to 58 grains, compared to 52-53 grains for the standard .243 Winchester."
