But if a thin piece of aircraft hull can stop that round, then wouldn't it be easy to create body armor (or even a lightweight shield) that's also immune to it?

The slug still delivers kinetic energy, and comes even in rifle calibers. Armor can probably be developed, but the spaceship skin could be armored as well. Any tech will generate defenses and then counters to the defenses. The goal to protect fragile areas would require the sci-fi equivalent of this round, not necessarily the over-the-counter variety available today.

Would not work if the combatants are armored. Fighting between armored combatants in an unarmored hull runs into a lot of problems, since anything that can penetrate the armor can also penetrate the hull. Probably best to try to localize and armor critical ship systems and have the crew don spacesuits (ideally of the mechanical counterpressure kind) prior to action. You could also try to have the spacecraft be self-sealing, but I wouldn't count on that working.

If trying to be realistic, armour would be limited by the cost of launching it into orbit, as it is relatively heavy gear, so you could argue that for most encounters aboard spaceships armour would be a rarity, only donned by high-end units, making said units ever more deadly.

The concept of a "safety bullet" is right up there with "military intelligence" or "solar powered candle" as an example of an oxymoron.

@Demi: even if hull weight is a priority, the weight/flexibility constraints on hull armouring may very plausibly be much looser than the constraints on body armour.

An accessible source of information on what bullets do when shot into or through things is The Box O' Truth, theboxotruth.com/the-box-o-truth-1-the-original-box-o-truth . I've linked to the first one, there is a lengthy series. The author shoots a variety of ammunition at different barriers and reports on what they do.

Well, if I was a space suited pirate trying to board your vessel, kill your crew and steal your cargo, how likely would I be to pack safe bullets? Maybe if you ask nicely? This answer is silly if you expect gun toting enemies shooting at you to use safety bullets in case they kill you by missing.

@Criggie: Indeed. My flesh is tougher than my walls. But a solar powered candle is more or less a thing. It runs at night powered up from the previous day.

@Innovine: They're more likely to pack safety bullets if they realise that otherwise they'll kill themselves...

@Innovine: Or perhaps they want to kill you so that they can take your undamaged spaceship?

@jamesqf punching a hole in the side of the ship is pretty easy to repair, just weld a metal plate over it once you've gotten rid of the bodies. I don't think it's much of a concern

Here's the rest of that quoted paragraph: "Air Marshals are now issued SIG Sauer P229 pistols with a 12-round capacity firing conventional-jacketed hollow point ammunition in .357 SIG caliber." The consensus on frangible ammunition seems to be No thank you. It's JHP or bust.

High velocity, hull-puncturing rounds are safer for the shooter (no chance of ricochet), more likely to penetrate defenses and armour, and depressurization isn't an immediate danger (and the ship will likely be depressurized due to cannon fire before boarding anyway).

These rounds do not ricochet. Part of the design. Not debating the validity or tactics of space combat, just providing an answer to the question asked.