How do you make a zombie crawl when you're playing nazi zombies on black ops?

I just bought black ops yesterday so I can play nazi zombies. At the end of every round we wanna blow the legs off of a zombie so it can crawl while we repair windows and ge guns. For some reason we can't get blow the legs off of a zombie as easily as you could in in COD: world at war's nazi zombies mode. Help me out. I need to get a crawler ASAP