Assault rifles and machine-guns are often used as wrecking tools in urban combat, mainly by firing a lot of bullets at one spot. The U.S. Army has conducted experiments and determined how many rounds of 5.56mm (M-16) and 7.62mm (machine-gun) rounds are needed to penetrate various materials, and then to blast out a 6-7 inch loophole that you can fire through. For example, an 8 inch reinforced concrete wall takes (on average) 35 5.56mm rounds to punch a hole through, and 350 rounds to make a loophole. With 7.62mm rounds, it takes about a hundred rounds to make the loophole. A cinder block wall, with one layer of brick on the outside, requires 60 5.56mm rounds to get through, and 250 for a loophole. For machine-guns, it takes 30 rounds to get the loophole, and 200 rounds to make an opening large enough for a man to crawl through. Troops are constantly reminded that 5.56mm rounds will go right through interior walls made of thin wood paneling, sheetrock, or plaster. It's also important to remember that hiding behind a car will protect you from 5.56mm rounds, but not machine-gun bullets (or any 7.62mm full power rifle bullets.) Same with wood frame building. Most 5.56mm rounds will be stopped, but not full power 7.62mm.