After you jumped out of the window in the burning building in
chapter 6 you will see a big gate at the end of the narrow path. Look underneath the mattress that's by the window and pick up the MG42. It comes with 250 ammo and can take out quite a few enemies.
Walk to the gate and take out as many enemies as you can during the next few corners. After the MG42 is empty, reload the checkpoint and repeat. The kills add up and the MG42 will be waiting for you.