Simple Squib (gun wound)
Two simple solutions:

1.) Plastic bag of blood with a fine wire/string stuck to it.  Thread the wire through the wound hole in the shirt/leg/buttocks of the clothes, and yank.

2.) Lightly compressed air with a blood hose.  Indy Mogul's first Backyard FX episode has a great video on building this kind of rig: http://www.indymogul.com/backyardfx/episode/BFX_20070611