Of course there are realistic guns in destiny. We still have guns that were used nearly 100 years ago. We will still have these in 100 years. We just don't currently and probably wont ever use them. Plus.. its hard to be creative without creating the same thing over again. So using ideas and designs that are actually there, is much easier. Just like HoW.. They are using weapons they have already designed. Changing them a little and making them usable. (New fallen guns) they look alot like the guns used by the fallen. That's just Bungie's creativity.