There's a video floating around showing a four-shot cell-phone gun. Don't remember whether it was rifled or not. Let's face it, in that kind of close-up application, you don't need rifling. Like the Liberty pistol, which was smoothbore.
A .25 cal flare? That's pretty small.
Penguin used to make a .45 ACP cartridge loaded with a pellet of flare composition about .25" diameter and maybe .5" long. Worked pretty good, just kind of loose in the case, sealed with (I guess) green silicone goo.
Worked pretty good, going up about, what, 150 feet? Certainly enough to attract attention in an emergency if someone was flying around looking for you.
Somewhere in my dusty collection of thousands of photos I have a pic of my then-wife shooting one and the trace is pretty bright, even in daytime.
Penguin also used to load tear gas (CN? CS?) into .45 ACP cases. I think they were intended as last ditch crap-in-your-pants bear medicine, supposedly giving the bear enough pause to plunk an aimed shot in, I guess, the eyeball or something... never saw much use for it, but I fired one in a small room just to see what it was like.
The tears were only a side effect.
There was a small charge of (I guess) Bullseye behind a silcone wad, then the tear gas "microdust," they called it, then a top wad of silcone.
I don't think they're in business anymore, though.
I started to look into 12-gauge flares for my Mossberg about two years ago, but the only ones I found were custom made for the company's flare launchers. They might work in a break-action shotgun, but they were too long to chamber from the loading port of a pump or semi... you'd have to take the barrel off, insert the catridge, then put the barrel back on, a practice I would only recommend to Satan or somebody equally likeable.
I put the project off while I decided whether it was worth getting a regular flare launcher for FHTF situations.
Still deciding.