Easy CO2-powered Squib for Self-contained Blood and Gunshot Special Effects

Step 8 Triggering the Effect

(Sorry no pictures yet.) When the time comes, all the actor has to do is casually reach into his pocket (or wherever the trigger is), flip the safety off and squeeze a short burst on the trigger. Blood goes everywhere. The actor falls down and plays dead. Cut, print, and that's a wrap.
It only takes a very short burst to make the effect work. CO2 expands very energetically, and is stored at very high pressure. A little goes a long way.