[Rocky Raab]

How different would it be from a standard Rem 8mm Magnum?

I'm don't know what the two case capacity numbers would be (in either grains of water or percentage difference).

But, in general, you can use this rule of thumb: For the same caliber, with the same bullets, the muzzle velocity will change by one fourth the percentage change of case capacity.

As an example, if the 8 RUM case holds eight percent more than the 8 Mag, you will see a two percent increase of velocity.

If the 8 Mag velocity is 3000 fps, the wildcat 8 RUM would churn up a whopping 3060 fps with the same bullet.

Edited to add: I just checked. The 8 RemMag holds 98 grains of water. My list didn't have any RUM rounds, but the 338-416 Rigby is very similar. It holds 106 grains of water. The difference is 8 grains, or a nine percent increase.

Expected velocity increase of an 8mm-416 Rigby over a stock 8mm RemMag would be one fourth of nine percent, or about 2.3 percent.