I worked on a boat called the California Dawn waaayyyyy back in the 80's. 1985-1988. We sportfished sharks, and learned a LOT about shooting them.
1. A 12ga. is quite effective. To note, if'n you use buckshot, you are likely to blow the jaws apart depending on the writhing of said shark and shot placement. And you will see the customer's face go ashen as his prize mako jaws rain tooth particles down into the blue.... and your tip goes up in smoke!
2. 22LR CAN do the job, if you hit the shark in the right spot. If you miss the brain, you just zip tiny holes in the shark, and the shark does not care.
3. Best rounds we found were lightly built 9mm-44mag. They penetrate a good 3-4" and then break apart, sending all the energy into the brain cavity. Teeth are saved, shark is rendered Jello. Ball ammo in either caliber tended to go thru the shark, and often did not KO the fish like you wanted.
As to spearing them, I have speared makos, threshers, soupfin, and leopard sharks for food. I put a spear in a very large and aggressive tiger in Hawaii in 2004. I have had to spear bulls in Hatteras, and Tx. The bulls tend to not take 'NO' for an answer sometimes.
When SPEARING a shark, what we have found is:
1. 223 bangsticks kill every shark you hit with them, so long as you hit the forward section. If you hit the rear section, you paralyze the shark. And then he gets eaten by a much larger and less friendly shark. And you wonder WHY on EARTH did you not bring a second 223 round to help you with your new found friend....
2. When spearing a shark for sport/ fun, I try to spear them sideways thru the gills. The perfect shot goes into the gills, and is angled forward a bit. This cracks the cranium, and paralyzes them. Lights out.
3. Sharks which swim in a slinky fashion like leopard, blue, dusky sharks tend to roll a lot when speared. They will twist up your shafts something terrible.
4. When tigers roll in, I roll out. The fact that they can get to 18ft or so and EAT people- not just bite people puts them in a special class with big bull sharks. I do not look for a conflict with either. I just leave.
5. Being in Ca....... all I can say is: "There is no such thing as white sharks. They do not exist. They do not eat seals in the same water I swim in. White sharks are like Bigfoot. Someone must have seen one, but they are not real."
After dropping down in very dirty water near Bodega bay looking for halibut on the bottom in the murk, I surfaced to the sound of a boat. There was a DFG warden looking at me, and he was perplexed. "Are you
really diving out here?
Here? Now?"
'Yeah, the halibut are in. I have 2 good ones in my kayak.'
"Take a look over the other side of your board...."
And there, still turning and gasping for breath, was a bull sea lion, torn in half. Still breathing a bit. And bleeding up a storm. The red cloud was maybe 12ft. across.
I contemplated it, and to ward off my impending terror, I smiled and told the warden 'Oh; Good. At least he's eaten, and now he's not hungry.'
I dove, and right there, right under me was halibut #3. About a 20lb fish. I shot it, and swam the 30 feet back to the surface, and took a breath. I climbed onto the board, pulled the fish up, and slid it into the hatch. I told the warden "Well, that's my limit. Want to check my catch?"
He shook his head in utter disbelief, and slowly drove off to check on the H&L fishermen. And I was FROZEN with fear, but was glad I did the dive. The shark did not come back, I never saw it. Mr. Sea lion was just bobbing away, like a bleeding cork on the water. I shivered for a good 1/4 of my paddle back to the harbor..... and not because I was cold.
Of course in was in a petting tank at an aquarium. The employee promptly announced, by microphone, " No grabbing the sharks!" 
