Careful. Capacity should be limited using the minimum cartridge length (2.655) not the maximum (2.76)
Capping the mag at 5.9 round of longer 3inch shell (2.76) would allow 6 rounds of smaller 3inch shells (2.65)
5 shells being 0.1 inch shorter would give you an extra 1/2 inch of mag capacity pushing your 5.9 x2.76inch to about 6.14x2.65