Tell you what, Red Eyes, just add up all our answers and divide by three and hope no physicist reads your book. 😉

I like the creativity in the question, but fail to see the worldbuilding aspect. I've voted to close. I would suggest another stack exchange if there is one suitable.

@Trioxidane Is there a Stack Exchange for Pineapple and Pineapple-adjacent topics?

Keep in mind that the launch converts force into velocity, and the hit converts velocity into force. The mass of the object does not matter - a heavier object will move slower but will hurt you more (per unit of velocity) and this cancels out. Barring physical shape of the projectile (e.g. sharp bits) or significant differences in air drag, the spud gun will hurt you as much whether it launches a spud or a pineapple.

@Daron: there is Physics.SE and MedicalSciences.SE, additionally to Biology, Engineering...

The speed (alone) of the coco-apple at the exit of the gun is mostly irrelevant. There are other things relevant, and they are at the place of the impact: velocity, energy, mass, size, pressure, hardness of bone, which exact place of the skull is hit...

@Tortliena: on Physics.SE you could find about speed, pressure etc, on Medicine or Biology about deadly forces. And you could just link the questions to each other. You, or OP, or whoever :) And yes, it is a multi-disciplinary question.

@virolino We shall not post questions across multiple stack-exchange, even if to link them latter (can't find back the help-center reference). Doing so does not make them more fit for another SE, on the contrary ^^. Indeed, why is it here if it's in another discipline 🦋?

Time flies like an arrow. Fruit flies like a pineapple.

You can use haloween turnips for a similar, albeit creepier, result.

This would be the most meta way for the next Psych movie to kill someone. I'm extremely invested in the results.

If you're allergic to pineapples, maybe not very fast at all.

Very poorly defined question. What's lethal? Lots of non-lethal things will, in certain circumstances, kill people.

Late to the party, but isn't the question phrased as-is offtopic for this SE? It feels like it belongs on Physics, or Engineering(as has already been mentioned).

Slightly larger than throat sized, shoot into the throat. Doesn't need to be fast at all.

Never let it be said that politicians don't encourage scientific inquiry: businessinsider.com/…

Today on Stackoverflow: Advise for would-be murderers on how to best confuse the investigators. "He was killed by WHAT ?"