Rotation. Bullets spin, it’s for stability and range. Weapons are built to spin the bullet while it exit.

Non round shots would not be able to spin at the same way due to resistance and aerodynamics and will be much less effective.

Yes. Square bullets would catch the wind and fly in unpredictable directions.

They spiral the barrels of guns to get the bullets spinning and more accurate.

A shotgun slug can be anything you want.

The puckle gun had square bullets designed for Muslims and regular ones for Christians

https://en.wikipedia.org/wiki/Puckle_gun

Puckle demonstrated two configurations of the basic design: the first, intended for use against Christian enemies, fired conventional round bullets, while the second, intended for use against the Muslim Turks, fired unconventional square bullets. The square bullets were considered to be more painful and would cause greater severe wounding. They would, according to the patent, “convince the Turks of the benefits of Christian civilization”.

Dude are you on something?

other than stablised flight, ask your self this: how would you cut a square hole in a billet of steel vs drilling/boring a round one?

Shoutout to the Kraut Space Magic G11 and its weird, squared-off caseless cartridge. Note that the bullet itself is still circular.

This was supposed to help squeeze more propellant into a magazine by not ‘wasting’ the space unfilled when you stack circular cartridges on top of each other. In practice, the G11 had a number of technical issues, some of which could be traced to the caseless ammunition. It ended up going nowhere.

Aerodynamics.

I’m no expert but as I recall, when a cartridge is fired, the casing is going to try and expand due to the pressure as it fires.

A circle design is the most resistant to case expansion. I think any other shape will have weak points which will lead to misfires.

That and aerodynamics as (just about) any other shape likely drift out of control and negatively impact ballistics.

speak for yourself, I got my pucket gun ready for when Turkiye invades https://youtu.be/CQZ1Ybq6akI

also the G11 has square cartridges if that counts

Throw a block.

Then.

Throw a ball.

You tell me.

I just realized you meant a Minecraft-style square cartridge. There are a lot of very good reasons for this that should be very obvious to anyone who knows even a little bit about how firearms work and thinks about it for a little while.

Rifling, case expansion, aerodynamics.

First, some terminology. A “cartridge” is fully assembled and ready to fire. The “bullet” is the projectile that comes out of the barrel.

Some cartridges do have bullets with a flat nose, like the .40 s&w fmj cartridge pictured here. https://en.wikipedia.org/wiki/.40_S%26W

Some bullets are even “flatter”, like wadcutter rounds, which are designed to cut clean holes in paper. https://en.wikipedia.org/wiki/Wadcutter

In the case of handgun rounds, larger caliber bullets tend to be flatter to prevent over-penetration leading to collateral damage. Smaller-caliber bullets tend to be rounder (so called “round nose” ammunition) so they penetrate more, giving a greater chance of having the desired effect on the intended target.

There’s a LOT more to it than that, but that’s the quick and dirty answer. There’s also hollow point ammunition, which is a whole other rabbit hole, and we haven’t even talked about rifle cartridges yet, which can have wildly different bullets for all kinds of applications.

The reason why Glocks have that characteristically square slide is so the sights can be as low and as close to the barrel as possible. There’s a very long-winded reason as to why this is a good thing on a handgun. See the “bore-to-sight distance” section here for that: https://en.wikipedia.org/wiki/Bore_axis