I use CCI200 (standard) without problems. AR2209 is a bit slow for the 375 and you may want to try AR2208. Most of the 2208 loads will be slightly compressed - I'm suprised that none of the 2209 loads are compressed since it is a slower powder and you use more of it.

I've only used magnum primers (CCI) for cast lead projectile loads (270gn gas check and 30gn AR2205) since there was a lot of free space inside the loaded case and it resulted in hang-fires. A filler can be used to the same effect instead of the magnum primer for the cast loads.