Corvus,
Welcome to the forums as well!
As to your questions there are a lot of different ways you can produce these effects. For mud it depends on if you want the dried or wet look. For dried, regular old dirt mixed with white glue and painted works as does other techniques using pastels or weathering powders. For wet mud, you can do the same with the glue and seal it with a lacquer of Future to retain the wet look.
For the bullet marks, are you talking about scratches? If so, I use the toothpick method with thinned paint to nick/scratch/weather. As far as natural camoflage goes, most scale-railroad stuff works as does the real thing like dried/plastic foliage found in floral arrangement sections at places like Michael's or Hobby Lobby.
HTH.