just checked the rules @nrl22..The Air Rifle must be a .30 caliber or lower. Tethered tanks are not allowed. Projectiles must be mass produced, domed pellets. competing against 22 powder burners.just seems fair to allow us air gunners to compete against...and 30 cal is fair enough.
You can use slugs as well. Typically if you are not going to the nationals or anything like that, most MD's could honestly care less. Most it is about having fun and honing your skills.
Some MD might also have a weight maximum. This topic has been well hashed out in recent threads in the NRL PRS sub forum. If you are limited to 40g slugs I would use a 22Cal for the higher BC compared to a 25Cal.