Just stole this from Quora
Greg Taggart
Firearms trainer/educator, multiple certifications, research on firearms policy
A blank can kill you.
A blank is a cartridge loaded with powder but without a bullet. It is intended to make noise or simulate real firing, but without actually propelling a bullet. To allow pressure to build up, the blank is usually sealed with a paper wad sealed with glue.
When a blank is fired, a blast of super heated high pressure gas is discharged from the case and blasted down the barrel. Within 4–6 feet or so of the muzzle the blast effect of the superheated high pressure gas as well as unburned particles of powder can cause significant injury By literally blasting a hole in an unprotected surface. At Ft Benning it was fairly common to blow the head off snakes by firing a rifle blank at a distance of 6 feet or so, after the rifle blank adapter had been removed. Beyond the issue of hot gas is the glue plug- although light in weight as well as fairly frangible it is blown out of the barrel at near bullet velocities and will penetrate a soft target to a depth of several inches at Close range.
As for lethality? Depends on cartridge type, distance and where the victim is hit just like with real bullets.
In 1984, on the set of the TV show Cover Up, actor John Erik Hexum, while playing with a blank loaded revolver between takes, put the muzzle to his head and pulled the trigger. The blank shot caused massive brain injury and he died shortly after.
You should treat a blank loaded weapon as you would any other.