All the bullet textures are 2k and use PBR textures, so following standard PBR pipeline if you need to re apply the textures or want to port it to another 3d software. 

The materials set up within the blend file have a grunge slider, this changes between two texture sets.

0 = clean(no grunge)

1 = dirty(grunge)

i Recommend Adding it to your blender asset browser then you can just drag and drop it in when ever you need it.