A cartridge to never shoot in an encore

Several months ago we built an encore in 257 Weatherby for a friend. He was out of state for a good while and just came up today to pick it up. I hadn't been able to find any ammo for it so he brought a box of Weatherby ammo up. I normally don't build Encore barrels in a belted magnum but he talked me into it figuring it was a small bullet and the pressure shouldn't be too bad so I said OK.
Anyway we shot it here in the shop and the first 2 rounds blew the case in half. I should have stopped there but decided to try one more round. That round destroyed the barrel, broke the hammer in the gun and did damage to the action. And this was all with factory ammo.
Afterwards we checked everything, the barrel, the chamber, everything. And it all checked out. The round was just way too hot for an encore.