Cloth sims: There used to be an article by @EgoMoose on the old wiki but it got lost in the switch of URLs. You can find a PDF here still: https://drive.google.com/file/d/0B8NceKcllVYrXzhlaDFWdHp5YTg/view
Gun impacts: pretty basic raycasting. Putting a texture at the hit point rotated toward the camera or the normal.
Deformable snow: might use an early test of the cage mesh deformer but there are other ways. Ask the guy that made it 
Advanced AI: “when gun shot, play cower animation”
Any more details on any of these probably deserve their own post.