(Edit: In answer to AndersonG22) Multiple reasons:
1) Some people have a serious aversion to violence. For these types, it's often harder to make themselves use a stabbing, slashing, hacking, or bludgeoning instrument, so they are (edit: even) more likely to hesitate (edit: with a knife, as opposed to pulling a trigger).
2) While a handgun can be grappled (I was teaching somebody how to do that Wednesday night, in fact), the BG has to get to contact range to grapple it, whereas the shooter can fire before that happens; a knife (unless one is really good at throwing it) requires that the BG be in contact range, and a knife can be grappled. (Really, one grapples the knife user's wrist/forearm much more than the knife, per se, but there are also ways to grab the knife itself once motion of the arm has been limited.)
3) Depending on the gun, multiple shots can be put on target in the time it would take to get to the second effective attack with the knife.
4) If the BG turns out to have a firearm, he can retreat and shoot through the door, at the knife user.
(Edit) 5) Even trained knife users often find they've cut themselves after a knife is used for SD. Cuts to the fingers of the knife hand from sliding over the guard, to the blade, are common. Sometimes, inadvertently self-inflicted knife cuts are pretty bad, as in ligament, tendon, nerve, or major blood vessel bad. Untrained person with a knife?
In a scenario where a relatively untrained person expects to get into a contact distance problem, I'd recommend a hammerless or shrouded hammer (S&W Centennial or original Bodyguard and descendants) revolver over the knife. The short barrel makes it harder to grapple (although even with guns, my first attack is on the wielder and his hand/wrist, not the gun itself), and it won't get pushed out of battery like a semi will if shoved into the BG.
That is probably the ONLY scenario where I'd suggest a J-frame for a relatively untrained user. For others, I'd recommend a 4" DA revolver, or a service size auto, and at least some training.
(Edit: To clarify, I ALWAYS recommend training; I just realize that in some cases the threat exists before good training becomes available.)
Last edited by MLeake; January 13, 2012 at 05:52 PM.