.22 mag obviously has quite a few fans, and I wouldn't try to talk anyone out of using a gun and/or a round that they really like, but . . .
Using the Genitron.com site, I find 34 handguns when searching .22 WMR, of which just 2 were pistols, a half-dozen derringers, and the rest revolvers.
Searching 9 mm caliber gives 538 choices, including a handful of derringers and revolvers (all Taurus), the rest pistols.
Does that make .22 mag a "niche" round? I suppose it depends on your definition, and perhaps whether you are considering pistol or revolver.