Below is an extract from the NSRA Rules:
You will see that 10m air pistol will (normally) be restricted to .177 but .22 may be used for other competitions. So, if you shoot 20 yard air pistol you may use .22. The Competitions Handbook is the best source, if you look at Competition 270 - Club Pistol you will see:9.2.1.3 any type of compressed air or CO2 pistol calibre not exceeding 5.6 mm (.22 in) and of dimensions not exceeding those in Rule 9.2.1.1. The trigger must support 500 g (1.1 lb) weight; or
9.2.1.4 any air pistol complying with Rule 9.2.3; or
9.2.1.5 any type of muzzle-loading pistol or revolver of 5.6 mm (0.22 in) calibre.
9.2.2 Pistols used in competitions under ISSF or other rules must conform to the specific regulations governing those competitions.
9.2.3 Air pistols used in competitions under ISSF rules may be any type of compressed air or CO2 pistol calibre 4.5 mm (.177 in) which conforms with current ISSF regulations.
Less well known is the fact that 20 yard pistol targets should all be scored with a 0.22A gauge. It’s the same for LWSR air\rimfire.0.177/0.22 air pistols may be used. Free pistols may be used.
Rutty