Spicy 9mm? I think you mean Cop Killers.
"vests are optional" was always a horrible line to me, as if every single crook you'd run into would be rocking that 9mm AP ammo. Imagine dying needlessly to some unmedicated boomer with a .45 because you assumed your vest wasn't going to catch any bullets at all, no matter where they're from.
I finally looked up what "dumdums" are today. Just hollowpoints...what a letdown.
Yep, early, often improvised hollowpoint/softpoint style ammunition. Kinda works with high velocity rifles like .303 British, 7.92mm Mauser, .30-06 Springfield, etc, while being pointless at best with most handguns of that era (because making a good expanding projectile at low velocities like those is often difficult).
more specifically Dum Dums were rifle bullets that had the tip of the projectile cut off to expose the lead for expansion. they are much more similar to what are called soft points nowadays.
View attachment 2828166
There's also the stereotypical split cut or X cut tip.
In this old German propaganda from WW1, we can see projectiles they are accusing the French of using. Plain turning the bullet around is another method, which the fourth one shows (I think), and then there's the third one, which appears to have material cut out around the middle, probably difficult to improvise in the field.
Given that these loads of 8mm Lebel used solid brass projectiles, I wonder just how well this would work.
All in all, modern factory made hollowpoints and softpoints are a million times better than these old improvised ones.