Frankly, I wouldn't consider a 22A a very good gun for silhouette, but on the other hand everyone's got to start somewhere. Really, though, if you're going to shoot conventional/unlimited in pistol silhouette, you ought to try a S&W 41, Ruger Mark II/III, or (if you can find one) a Hammerli or Citation.
I shoot a Thomson Center G2 Contender in the small bore hunter pistol class and use a Nikon 2.5-6X 32mm (I think) with extended eye relief. If you don't want to use the taco hold (and with a semi auto depending on how your pistol throws its cases and whether you are left- or right-handed you might not want a taco hold ) then I suggest looking for one of the scopes with extended eye relief. Nikon makes them; so does Bushnell and of course Leupold. You can hold the pistol out as you ordinarily would and still see through your optic.
The fixed magnification pistol scopes with extended eye relief are a lot cheaper than the variable magnification scopes. Still, with a 21A you'll end up paying almost as much for a good scope as for the gun itself.
Open reticles of the red dot type: I've not had good luck with any of them, even a fairly expensive one. The first one the dot was so faint I couldn't see it in daylight. The second one took three shots and had the reticle come out of alignment from the recoil shock. It can't be fixed. So can't recommend them based on my experience.
I've never used a tube-type red dot, but the guys who win with those use UltraDots. I gather that must say something about the UltraDots. But again, no experience there.