I know I don't need to say this, but I will anyway
Any filmed scene involving any kind of discharging firearm, real or not, has the potential to create fatal accidents.
A BB gun has the potential to kill at short ranges!
Even a blank firing pistol can kill (Brandon Lee died this way when a sliver of metal casing was forced out by a blank cartridge, during a professional shoot)
To film any firearms sequence safely, really needs the professional help of an armourer.
However, that isn't practical on your shoot, therefore you need to find ways of bringing the risks down to zero.
The easist way to achieve this is to never have a loaded weapon of any kind on set. If you frame the shots in tight and add the right sound fx in post production, you'll get a fairly good and completely safe scene.
To achieve this, you have to personally check every single weapon on the set before each take, to ensure than nobody has loaded them.
The second rule is, even with an empty gun, nobody from the cast or crew is ever placed in front of the gun. In fact, ideally, you need everyone behind the firing line at all times. I know that this restricts your shooting options, but no film is worth anyone getting injured.
The final thing you need to do is brief the cast about the safety rules at the begining of the day and make sure that they are strictly enforced.
Safety on set will be your responsibility, in the heat of a production it's not unusual for actors to put themselves at risk in order to get the shot. People will take risk that they wouldn't in every day life. For this reason safety has to be the number one thought of any director.
One of the things you might consider doing, is a risk assesment for the scene. This is where you right down every single way a person might get injured doing the scene and every single hazzard. You then create working rules that will remove the risk as completely as you can.
Hope this helped. Have fun doing the scene.
