system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are there so many gun models using the same calibre? Doesn't the bullet determine most of the lethality and range, while the gun is just the vehicle that fires it?
gpt
The cartridge itself is the main factor in terminal ballistics, but barrel length is also important - a longer barrel gives the bullet more distance to accelerate, to a certain degree.

That aside, there's also plenty of other factors in a gun. Ergonomics, rate of fire, price, the action and lock type, materials, safety, other features to improve reliability or ease of use, the list goes on. Also, some models may be updated versions of older designs, direct competition to a similar model or just an improved but more expensive luxury version.

Think of it like there being lots of models of cars that use the same fuel.