[KoRn]

"A semi-auto should have the mag removed and the lock shackle pushed from the breech end of the magazine through the grip"

My H&K USP Tactical has a factory locking system where you can use a specially made key that comes with the pistol that you can use to turn a lock inside the grip that prevents the trigger and hammer from moving AT ALL. Just a turn of the key literally locks the weapon. Grant it, in a self defense scenario having to remove the magazine, find the key, turn the lock, and replace the mag/chamber a round would take far too much time to protect yourself from an intruder.

[Myrddin]

The new series of S&W revolvers has an internal hammer block as well. Those are pretty effective. For revolvers that predate that approach the padlock over the backstrap is pretty foolproof.

This model differs from my old favorite, but has similar 5-button lock features.