Just cooked another Costco Prime brisket. It was 13 lbs. untrimmed. Took off about 3 lbs of fat. Let it come to room temp for one hour. Smeared it with French's yellow mustard and then kosher salt and large ground black pepper. Placed it in a preheated 025 set at 275 degrees. Inserted a temp prove from Thermoworks. Approximately 3.5 hrs. later internal temp of 185 was reached. Pulled brisket, wrapped in butcher paper and replaced in 025. At a total of 5hrs. and 22 minutes, internal was 203. Pulled brisket still in butcher paper and wrapped in tin foil, and placed in 180 degree holding oven to be served later. If it is not juicy and tender I will report, otherwise consider it fabulous.