Kevin Wyatt at Wyatt outdoors makes about 4-5 different mags that will work.
I just use one of his center feed 4140 CM heat treated mag on a .284Win in a Stiller Predator SA. It required my welding the back of the mag box together. I used two 1" pieces of IV tubing between the action and the mag lips. Feeding the magazine from the bottom with 3 rounds it feeds like a champ. a 6.5 x .284 should be the same.