Surprise - Surprise .. ! ! This rifle is NOT a 30-378 ...
it's a 30-338 win mag ... see picture ... (The seller said it was a typo, and gave a partial refund for his error.)
The question then is; why wasn't this rifle just made as a 300 win mag? (which actually has 5 grains more case capacity) ... Well, ... It took some digging to find out why ... This rifle must have been built in 1958, when the 338 win mag was first introduced. The 300 win mag wasn't introduced until 5 years later in 1963. So, if you wanted a 30 caliber magnum in a long range match rifle in 1958, you simply necked down a 338 win mag. The only other 30 caliber mag available in 1958 was the 300 H&H mag, which wasn't considered a target cartridge.
Now the next question is; "Why did Dunlap use a Brevex Magnum action for a standard length cartridge?" In 1958 a surplus mauser could be had for about $10, and a Brevex sold for $118.50. That remains a mystery.
http://www.buckstix.com/buckpics/30-338caliber.jpg