I've run just about everything on the (Vancouver at least) market through mine. I find that accuracy does not vary enormously with different types of ammo. With a .22LR pistol, most people shoot at such short distances that all the drift, drop and voodoo associated with .22LR rifles doesn't come into it.
Subsonic ammo is supposed to be more accurate (at least from what I have been told), but I haven't found this to really hold with a P22 at pistol distances. Even really primo stuff like CCI "Greentags" seems to make no difference on accuracy. CCI "Standard Velocity" is very good quality sub-sonic ammo (usually at around 4 to 5 cents a round), but I don't find that it is really any better on accuracy.
Cheap 3 cents per round stuff (box of 500 = $15) does seem to be a little less accurate regardless of brand (Remington, Federal, American Eagle, Winchester, etc.). I think the best of the cheap stuff is those 525 round boxes of Federal "Champion" that they have. Remington "Cyclone" is not too bad either. I like it better than "Thunderbolt" and (especially) "Gameloads". American Eagle branded stuff generally sucks in .22LR. The Federal stuff under the "Federal" name is better if you can find it.
Try some higher velocity stuff out too. I have found CCI "Stingers" to be as accurate as anything with the P22. I was shooting some yesterday with a P22 and reminded how accurate they are. They also make a cool ultra-sonic sound and give off a muzzle flash. The bad news is that they are $6 for 50 (12 cents each).
CCI "Mini-mag" and CCI "Velocitor" are also good. Velocitors, I believe, are the heaviest bullet weights of the bunch. Remington "Yellow Jacket" I find to be pretty average. It is nowhere near as "cool" as the "Stinger".
Ultimately, the P22 isn't supremely accurate with any ammo. I find that if you really concentrate on what you are doing you can get down to loonie/toonie sized groups at ranges in the 8-10 metre range. That is damn good shooting with a P22 though. A fist sized group is more normal.
