Didn't think alot about this untill a phone conversation this evening, just another photo on the net of a blown up gun. But it turns out the 257 STW (AKA the 25 Hot Tamale) is a necked down 7mm STW, which is simply the 8mm Remington Mag necked down to 7mm. The 8 Remington Mag of course is the 375 H&H case given the good 'ol AI treatment, at least for conversational purposes. This means the 257 STW is a 95 grain case. That beats the 257 Weatherby by at least 15 grains. Then we are told that the powder used (least to best of recollection) was Reloader 19. This is akin to using 2400-4198 in a 308. To put it in perspective, Accurate Arms lists 75 grains of 3100 as a max load with a 120 grian bullet in the 7mm STW, 95 grains of 8700. To get to the 257 STW we neck that case DOWN which effectively increases the chamber/bore ratio, which requires a slower powder.
R19 and 4831 are in the same speed range, 3100 a bit slower than those. 8700 is about the right place to start. Retumbo should be about the powder of choice. I'm not going to guess what may or may not have happened, I wasn't there, and it weren't my gun. But if what we are told about the powder is correct, without doubt the powder was too fast.