With nothing better to do than ponder, I started looking at the gaps between the different common calibers. I was surprised to find that the jump from .375 to .410 was the greatest. With so many calibers you would think that the medium bores would be sufficiently covered. If another caliber is in there then it must be pretty obscure. So, now comes the question. Do we need a medium bore rifle with a caliber between the .375 and .40 calibers? Perhaps a .390 caliber (9.9mm) is in order.

Caliber ::::::: Caliber Gap
.223-------------.020
.243-------------.014
.257-------------.007
.264-------------.013
.277-------------.007
.284-------------.024
.308-------------.003
.311-------------.012
.323-------------.015
.338-------------.020
.358-------------.008
.366-------------.009
.375-------------.035
.410-------------.006
.416-------------.007
.423-------------.006
.429-------------.023
.452-------------.006
.458-------------.010
.468-------------.006
.474-------------.002
.476-------------.028
.504-------------.005
.509