Well Walt,
In NRA or Olympic matches .177 cal. is mandatory !
But in the GTA matches either is accepted. As stated above, the .22 cal is better at longer distances. I also catches the line edges on close shots that would be missed with a .177 cal pellet due to its larger diameter.
As for pellet, try out as many as you can, to find the most accurate in your air rifle or pistol.. I have found that sometime the cheaper pellets are more accurate than the big name expensive target pellets, in my air guns ?
Checking your pellets visually and weighing them helps greatly, when you have the time and desire.
I hope to see you shooting here soon !!!
Bill