a fifty ball plinking into your armour is going to flick you away like lint, it may not penetrate, but being booted over the next county might not end well.
I'm thinking spalling... your tummy is going to have the wobbles as is every cavity of your body. The Ke is roughly the same as a 300lb mass running into you at 100kmhr, with a pointy stick that is 1/2" across. The bullet may not penetrate the vest, but the vest may penetrate the person wrapped around the bullet. gonna leave a bruise.
the 50 has about 18-20kJ of kinetic energy. per standard
Your energy calc is erronous, 20kJ is 110lbs@100kmh or about 2 large dogs@1100 inches/second . . . probaby knock you over . . .