Chances are you've seen this effect in action or war films. A Character gets shot and their chest rips open spraying blood everywhere. In Hollywood, this effect is almost always done using a small explosive charge called a 'squib'. Basically, the actor has a metal plate fixed to their body and the explosive charge is attached to this plate and then a bag (or condom) filled with fake blood is taped on top. Detonation is done electronically and when the time comes for actor to get shot up, a special FX technician would flick a switch and BANG. Blood everywhere.
This effect is very impressive but has a number of problems. Firstly, using explosives requires a certain degree of expertise and might cause serious injury if done wrong. They're also very expensive and complicated. Unless you've got a 5 figure budget don't consider using explosives to simulate bullet hits. I've seen countless YouTube videos where people have taped firecrackers to their chest with a blood pack on top and then lit the fuse. First of all, you'll see the smoke and sparks through the shirt as it burns down and second you can't judge how long it'll take before the thing goes off. This method just doesn't work well and if you live in an area where fire crackers are illegal then you can't do it anyway. The method I use is cheap, safe, simple and best of all looks great if done correctly.
5 years ago I watched Peter Jackson's Bad Taste and was amazed. Peter Jackson (the very same guy who directed Lord of the Rings) worked on this film for four years and on a shoestring budget. It contained a lot of impressive special effects including dozens of blood splatter. I spent a lot of time trying out blood effects and I found this brilliant website showing you how to do a blood spray effect using a compressed air pump. Unfortunately this website no longer exists but there are plenty of tutorials around which explain this effect in greater detail.
You basically purchase one of those pump up insecticide sprays. If you're serious about film making you'd pay for a new one instead of using an old one that might be contaminated with nasty insecticide and could kill your actor. You then use a length of PVC hose that has one end sealed and a hole made on the side. This tube is then taped to your actor and the little hole is aligned with a cut in the item of clothing which they'll be wearing. Stuff the hole with small pieces of sponge to prevent the fake blood from coming out prematurely.
You then pump up the sprayer and I recommend you do it 150-200 times to get enough pressure. High pressure will ensure the blood sprays out like a cloud instead of just leaking out like a tap. You then attach the tube to the sprayer like the picture above and when you're ready press the trigger down and the air will be released and spray fake blood everywhere. I've tried this out several times with varying levels of success. The trick is to keep experimenting until you find the best combination of fake blood, pumps and tube position.
And here's the finished effect which I did for a university task. Thanks to my brother for allowing me to shoot him.