Unity 3d Bootcamp: Collision between bullets and barrels.

I'm trying to reverse engineer the bootcamp project in Unity. So far, I was able to import the soldier prefab into my game and get him moving about. He can shoot and fire grenades.

The grenades seem to work fine (i.e moves objects with Rigid Body attached to them).

However, I simply can't get its bullets move anything.

I even tried adding some objects of my own in the Bootcamp scene. I could only move them with grenades...not with my bullets. I tried looking at the barrels and tires objects, but I don't see anything different in there.

My question is: What makes tires and barrels in that project, react to bullet fire? What am I missing?