Well, you are in luck, I was the TD for our school's production of Les Mis last year. First, I am assuming you are doing this on a
stage indoors, not outside in what would turn out to be a mud pit for us. Here is what we did for effects, we first had a
fog system-that we never actually built-but was designed, and would have worked great if we had time. We had a
HVAC duct induction fan at the end of our pipe that the fogger sprayed into, then we had a series of splits that channeled the
fog to places around our
barricade to have
fog in random areas. We would then run bursts of
fog through it for a not to bad gunshot
effect, and since some pipes were longer, there was a delay as to when the
fog left certain spaces. This was a wonderful visual
effect, but you have to keep distance between the fogger and the fan, but the fan will suck all the
fog up, so you don't have to worry about a seal of some sort.
Small lighting flickers add a nice
effect as well, but are not needed.
As for audio effects, hinged 2x4's would work great, have your
stage crew behind your
barricade, or off
stage slapping them together at random intervals, the more the better. Handles on the 2x4's are very nice, and they are easy to operate and hard to break. For our show, we had live music, so we got our pianist to use a keyboard to
play gunshot effects off the keyboard into the sound
system, then into the
house. It actually sounded pretty good, and since he could see what was going on, more lifelike than recorded sound effects.
Mic-ing the 2x4's would be fairly difficult, because you would run into peaking fairly easily, and might sound bad in your sound
system, but it is something you should test.