Thought I would share this since it does exceed the .375. This is my 400PDK. It is .411 caliber made from 06 cylinder brass left at 2.65". Shoulder is .46" and 40deg. Resized capacity is 86grs of water. Kind of a correctly chamber 400 Whelen Long. Using the 2.65" and Hornady 400gr DGX in the cannelure the OAL is 3.34. The second case from the left is the 400PDK using 300gr and the second from the right is the 400gr. The others are the 338-06, 375 Whelen and 375H&H.

Using powders like IMR 4895 and RL15 the 300s move over 2700fps the 400s over 2300fps with accuracy hunting loads for the 400 at 2257fps. I've had trouble finding 300s that handle the velocity but haven't tried the Barnes or Woodleigh.

I used a MKX action Douglas 24" barrel with Whitworth style express sights. Then added a VERY dense piece of Bastogne. I think it turned out great and is now one of my favorite to shoot.