system
Given the following post, respond as if you're commenting on Reddit:
human
[General Sci Fi] The Ballistics of Coilgun Small Arms

I apologize in advance if this isn't the right place to ask. I saw an artist's personal Sci Fi setting on Tumblr the other day, and one of their characters carried a handgun that used coilgun mechanics (rather than gunpowder) to accelerate small 2mm bullets. Their thinking was that a small round without a casing, propelled at high speeds, could preform similarly to a conventional round and allow for very-high capacity weapons.

That got me thinking about real-world ballistics, and the debate between smaller, faster rounds, and bigger, slower rounds. What are people's thoughts on the properties of a high-speed (lets say comparable or slightly above modern weapons), very small round against armored and unarmored targets?

EDIT: To abide by the Watsonian criteria of the subreddit's rules, I'll clarify that this applies to both terrestrial combat (planetside) and boarding actions on spaceships, where the character was depicted. I was under the impression that Hi-Velocity firearms and livable, pressurized spaceships wouldn't mix well.
gpt
iirc, the speed of a projectile is the most important part of penetrating armor. Small and fast rounds would be perfect for powerful, armor-piercing sidearm. But a larger bullet has more inertia and therefore is more resistant to changes in velocity, so long-ranged weapons like sniper rifles should be firing larger projectiles.