It would be hard to double charge a large capacity case such as the 257 STW given the slow powders required. I suspect the wrong powder, wrong charge of the correct powder, ....? This is a very high pressure cartridge well in excess of 60K psi. The slightest variation in components when working at max or near max loads could have taken it over the top for an Encore. This might not have happened in a strong bolt rifle. I love the Encore idea and have made several bbls for them in wildcat cartridges but I believe the big over bore mags/high pressure rounds are best left to the bolt or Ruger SS type actions. IMO... Many of the custom after market bbl makers won't chamber in these high pressure rounds. I have see 2 stretched/sprung frames both in WSM calibers. This guy should have bought a lottery ticket that day, he is one lucky shooter!