The '06 IMP case has the same capacity as a standard 9.3x62.

One chart I've seen, had the shoudler of a 9.3x62 at .450" to .452". That puts it .004 smaller than an '06 IMP, however - I've measured 2 different 9.3x62 rifle's fired brass and they both run .454" at the shoulder, which is the same as an improved '06 case.

There is a shorter neck on the 9.3x62 case thna an improved '06 with a 40 degree shoudler, whereas the 9.3x62 has a 17 degree shoulder, and the 9.3 also has a larger base than the '06. Both of these measurements, larger base by .004" pluss a slightly shorter neck combine to increase it's capacity to the same as an improved '06, whether it is necked to .358", .366" or a .375".

My first 9.3x62 used my old .375/06IMP brass form my first such chambered rifle, by merely necking them down to hold a .366" bullet and fireforming them. They fit perfectly and re-formed without case loss.

The larger bullet diameter of the 9.3 allows slightly higher velocities than the .358 - both virtually on the same case, with the .375 on that case delivering the highest speeds - same bullet weight.

My Oberdorf Mauser 9.3x62 delivered 2,675fps with 270gr. and 2,519fps with 286's from it's 22" bl.

My current .375/06IMP gets 2,740fps with 270's (sounds like H&H factory to me) as well as 2,470fps with 300's using Varget, H4895 and BLC2.

The Whelen .35 IMP should run 2,650fps quite easily with 250's - perhaps 2,700fps with careful loading of the correct powders. One MUST know what one is doing.