They are used in military combat to puncture engine blocks.
I'm skeptical, but in any case they weren't using them to attack car engines in the movie.
And now that I'm thinking about it, since they are representing his car as a BOSS 429, and since they built fake Mustangs to use in the first two movies instead of smashing up actual classic cars, why didn't they just build fake BOSS 429s instead of building fake ordinary Mustangs and pretending that this is what a BOSS 429 looks like?
There are even commercially available 12 gauge armor piercing shotgun rounds. Look it up.
I'm sure there are people who sell such rounds, but they would only boost a shotgun's armor penetration to the level of an ordinary non
-armor-piercing rifle round.
So why not just use automatic rifles to defend against the attackers (especially since they would get real armor penetration from AP rifle rounds)?