While ballistic gel and human flesh are different, the use of bg to standardize testing is completely accepted. FBI penetration standards are minimum 12" and maximum 18". Most expanding ammo from a defensive handgun does right at 12"-14".
I worked at Barnes Bullets and we set up a chronograph on the far side of a 12" ballistic gel block. Most expanding defensive ammo of any caliber 9mm-45ACP will now perform similarly to meet the FBI specs. We shot some 9mm Barnes TAC-X (solid copper, expanding) bullets through 12" of gel. Those that exited after full expansion were traveling less than 300 fps max, most much slower. Since they were already expanded, penetration at that slow velocity would be minimal, or non-existent. This exit velocity is also decreasing very rapidly over a short distance. BTW, a 91 mph fastball is going about 200 fps, but the ball having greater mass retains more velocity and energy than a little bullet.
The fear of making a decent defensive shot, having the bullet expand, penetrate deeply enough to exit, and then travel further and have enough energy left to kill an innocent is so very remote that it is of no concern to me.
If someone is worth shooting because he is in the act of killing innocents, shoot the bugger. Call Masaad as an expert witness if you need to, but you won't.
Rastoff has it right--document a case of a common self defense bullet expanding, penetrating completely through the perp, striking and killing an innocent down range.
Not rifles, not dogs, not missed shots, not special angles up or down, just plain and simple defensive handgun bullet over penetrating causing death beyond the target. Even if there are one or two out of many thousands of such shootings, statistically, it is not a concern compared to watching a lunatic (man, woman or child) kill person after person.
This is, IMO, one of these "let's sell magazine articles and books" topics that in truth is a non-issue. Hypothetically possible, sure. Reality? Not.
