Glock 20, 10mm may not be the best choice but i would like to have 15 rounds of hot 10mm over a revolver. Depends on where i am at mainly i cary the Glock 45 in 9mm. With the Underwood extreme penetrator +p+ it should be able to penetrate anything in North America. I say that somewhat joking, would rather have 10mm but i would say depends on where i will be.