having that bullet go in one side and punch out the other would probably ruin your day...
Update: Went to the Muzzle Energy calculator at
http://www.zvis.com/bvtengy.shtml
Using the Type P-41 7.62x25mm cartridge as an example, I calculated its muzzle energy. The P-41 has a 74gr bullet with an 8gr charge, and produces muzzle velocities (from the Burp Gun's 10.5" barrel, of about 1600 feet per second.
According to the calculator, it generates about 420 foot-pounds of muzzle energy. And this is military ball type ammunition. That's compared to 115gr FMJ 9x19mm, which generates 341 foot-pounds, and Hydra-Shok 124gr +P+ JHP ammo at 1220 fps, which generates 410 foot-pounds of muzzle energy.
Not bad, actually, not bad at all.