Food for thought as well as for the shark
How to turn valid objections in Plot Thickeners:
2. Human stomachs have acid in them that help digest food. I would think sharks do too. That acid may, if the timeframe is long enough, dissolve enough of the bullet to make matching impossible.
Agreed. Since sharks don't have molars to chew their food, they rely on a corrosive acid bath in their stomachs to digest it. If the bullet is a jacketed hollow-point (a typical handgun anti-personnel round), the copper jacket where the identifying barrel imprints would be may dissolve quickly even while the lead core could survive in acids for a while.
However, if the bullet passed through some obstacle - perhaps a tinted window - before hitting its victim, it could have tiny bits of non-dissolving glass embedded in the bullet which forensic study could find and link to the murder.
3. Great whites generally don't eat people.
True. We don't taste all that good; GWS prefer pinnipeds for their higher fat content. However, I've watched enough episodes of Shark Week to be convinced that they'll gobble up anything under the right circumstances. In a competition, a carnivore may wolf the food to preempt its consumption by rivals.
So, if the murderer chummed the water on purpose to attract the body-disposal crew before dumping the corpse, the first shark on the scene might reflexively chow down on whatever's there.
Also, does it have to be a Great White? Tiger sharks are well known for eating ANYTHING; it doesn't even need to be edible.
4. Great whites generally don't eat dead things...I think.
Yes, but... Plenty of predators will double as scavengers from time to time.
Again from Shark Week, I watched an episode depicting a GWS feeding frenzy on a bloated whale carcass. The sharks clearly were exhibiting scavenging behavior because, according to the narrator, the whale was well into smelly decay. The sharks gorged themselves into a food coma; it was like Thanksgiving at my house.
5. Fishermen generally don't catch great whites...I think they're a protected species.
Could a shark be snagged in a net as accidental bycatch? Suppose they unpurse the net and all the tuna and cod slide out but there's a terminally injured monster so enmeshed they can't get it free safely?
6. If someone is shot in the head, the bullet rarely remains in the skull...I think.
Correct. At close range, most bullets probably go right through.
However, one of the features of jacketed hollow point bullets is that their expansion helps to keep the round from overpenetrating. Also, if the bullet had been slowed by distance/obstacles it might stay inside the headbone. Or maybe it was a special load with a low-mass projectile for minimal recoil because the murderer is a small female; everybody knows they are more deadly than the male! (That's not meant as pejorative. It's another reason to be respectful of everybody at all times.)
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Al, I'm impressed with how quickly you can create these stories. It's taking me forever to write my own. Good luck!