The max length that will fit in the mag box, or shorter if the bullet contacts the rifling, which is not likely. Mine contacts the rifling at about 3.830 with a 200gr Accubond but my max length is 3.660-3.665 as anything longer will not fit in the mag box. BTW, the 200gr Accubond with about ~90gr Retumbo shoots awesome when seated to fit the box at 3.660". Max load is slightly higher though.