The twenty-two caliber rimfire shotshells that hold a tiny amount
of very fine shot #12 or so "dust" shot.
Traditionally, rat shot shells were all brass with folded crimps
(federal still makes them that way) .
CCI makes a .22 shot shell with a blue plastic shot cap that
shatters as it goes down the barrel; at close range, the plastic
shards will damage a cardboard box almost as much as the shot.
The Federal .22 shot shells are labelled "birdshot" i guess to
avoid people thinking they were getting buckshot.
Mossberg made a smoothbore .22 rinfire shotgun and a
clay pigeon trap set call "Moss-skeet-o" ising the rat shot
or snake shot ammo, but it never caught on.
Indoors, I would suggest a non-firearm approach to killing pests.
I did live capture a black snake in my mother-in-law's kitchen
once (used a mop to hold it down) and walked it out into the
woods a quarter mile before releasing it. The snake under the
clothes hamper had the color and patterns of a copperhead
and a hallway and two furniture cluttered bedrooms to escape
into, so I took no chances. I later filled all gaps around the
pipes and crevices with expanding insulation foam after that.
Whoever my mother-in-law hired to redo her bathroom did
not finish up the job properly for snake country. (Last night
I watched the "Crevices" episode of "Dark Tales of Japan"
only they had ghosts not snakes creeping out of the crevices.)