Again, there are 30mm DU rounds that melt through armor, if 20mm rounds can be fired with a shoulder mounted rifle as has been done, a 30mm could as well, would just need to be huge. And also, many of these walls are just standard building materials, they had AP rounds in the 30's that would go through steel plate. Modern .50bmg AP tungsten core rounds fired from a Barrett shoulder fired semi auto tactical rifle will go through inches of steel. So why then, given these FACTS, is AP rounds fired from shoulder mounted rifles in a game about the FAR future filled with alien tech that is much more far-fetched a problem?