Can slugthrowers penetrate stormtrooper armour?

Question

Many Star Wars fans claim no (including on this site, I see the claim often backed by no citation), as does one journal entry for Star Wars Rebels: Rebel Journal by Ezra Bridger (at least according to Wookieepedia's article https://starwars.fandom.com/wiki/Slug#cite_note-EBJ-2; any quotes from the actual book would arguing one way or the other be greatly appreciated). However, in Aftermath, Jas Emari kills many stormtroopers with her slugthrower.

Here are some quotes from the book (this happens several times in Life Debt):

• Page 332 (Jas is armed with a slugthrower):

  Already, they’re waving her toward the Halo. A pair of forest troopers comes up a long spiraling ramp—one is already on her, so she cracks him hard enough with the butt of her gun that his helmet spins. The other gets a point-blank shot to the chest plate. It splits in half and he goes down, his armor shattered and smoldering as he twitches.

• This is made more explicit on page 364:

  Jas takes down troopers left and right with her slugthrower. Jom is altogether more brutal—he and the Wookiee get right in there, scraping bodily with their foes, flinging white-armored incompetents left and right often into one another.

Answer 1

Yes, some slug throwers can.

"Slug throwers" as a category includes every single gun currently in existence on Earth. There's a wild difference in power between a 9mm pistol and a .50 sniper rifle.

I found a video at LiveLeak.com showing a range of different guns, from a 9mm pistol to a .50 sniper rifle, being fired at a "60 litre water filled plastic drum".

The 9mm puts a hole in the barrel, and a little water leaks out, but that's about it. Somewhat larger guns, such as the AK-47, actually lift the barrel off the ground and create large sprays of water. A smaller sniper rifle rips a large tear in the far side of the barrel. The .50 cal sends the barrel into the air. It lands on its side, nearly ripped in half, largely empty of water.

As a consequence of this range in power, real body armor is rated by how much force it can stop. Most "bulletproof" vests actually only stop handguns. From the Wikipedia article on bulletproof vests:

Because of the limitations of the technology a distinction is made between handgun protection and rifle protection. See NIJ levels 3 and 4 for typical requirements for rifle resistant armor. Broadly rifle resistant armor is of three basic types: ceramic plate-based systems, steel plate with spall fragmentation protective coating, and hard fiber-based laminate systems. Many rifle armor components contain both hard ceramic components and laminated textile materials used together.
...
Because of the use of ceramic plates for rifle protection, these vests are 5–8 times as heavy on an area basis as handgun protection. The weight and stiffness of rifle armor is a major technical challenge.

So, in your story reference, the fact that Jas can penetrate stormtrooper armor with a custom long rifle shows that at least some slug throwers can penetrate stormtrooper armor, but it certainly does not imply that any arbitrary slug thrower would succeed as well.

Answer 2

As far as I'm aware, Aftermath is considered canon.

So if a slugthrower penetrates Stormtrooper armour in Aftermath, then a slugthower can indeed penetrate Stormtrooper armour.

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Jas uses a custom made slugthrower, so it's feasible that this weapon could be more powerful than a standard slugthrower:

As a bounty hunter, Jas was a skilled sniper who wielded a self-made long-range rifle that was based on an old Czerka slugthrower. Emari was also good at hand-to-hand combat when circumstances demanded. She was also a good planner who was able to build an improvised map and plan using household objects. In her line of work as a bounty hunter, Jas had to kill numerous individuals

Although, I can't see if it's explcitly stated that it is. Main characters usually use custom or unique weapons. I'm sure there's a trope for that somewhere.