From memory: About a year ago (maybe 2?) here on the Big Island, one adult male shot another in anger with a speargun, aiming for center mass, nearly point blank range, during a confrontation. The spear shaft struck the victim just to the side of the sternum, and lodged with the tip and about 1/2 of the spear protruding from the victim's back. The shooting occurred at night in the victim's backyard, as he had some sort of argument with a "tenant" who lived in a trailer on their property.
He was able to walk back to his house, call for his father, and wait for the ambulance to arrive.
So, will a speargun, even a large one, "put down" a determined intruder? Only if you "stone" him. You are putting a small hole, probably through-and-through. If nothing "vital" is stuck, you might have simply really pissed off the intruder.
On the other hand, psychologically, a speargun pointed at me would probably scare the bejeezus out of me, more than a handgun would, even though "intellectually" I know the handgun is far more lethal. Just like a shot gun is more frightening and a better deterrent in many cases than a handgun.... it just evokes fear.
Hopefully this discussion will always be "theoretical" for all of us.
Best wishes.
This is actually also the arguement for powerheads...back in the 80's, when I was big into spearfishing, even a huge gun like my 60 inch double barrel Ultimate gun, would not necessARILY stop a big fish....more times than not, there was a fight on your hands...as we got to diving the 165 foot stuff, and then the deeper stuff in the early nineties, that "fight" with the fish was becoming a problem, because after your shot, you still had to do deco...maybe up to an hour...and if the fish fought with you too much, you'd have the bullsharks in, they'd be pissed, and demanding. And you would need to deal with them for the entire deco....
So Pat Frain comes up with these diposable, 9mm powerheads....slide one on in an instant, the spear point becomes the firing pin, the 9 mm round goes off anywhere in the head or near it, or the spine or near it, and the fish is in la la land....it is dead. Pat used to say it was far more "Humane" than shooting with a spear, and having the fish suffer for a long time.
The 9mm was great for up to about 80 pound fish....and then somewhere around here, you would want to move up to Pat's larger rounds, with even mo0re concussion..like the 223, or the 12 guage, or the monster load, the 444 marlin....
So there is no comparing the concussion effects of the 9 mm or larger rounds with even a big spear.....
On the other hand, this began in a discussion where the speargun was the only "weapon of opportunity" , in a Bruce Lee kind of sense....If you have an enemy coming at you with a deadly intent, you make the best use of the items around you that could be used as weapons....The speargun would be much better than your begging the bad men, not to hurt your wife or children.