You can get .22 barrel liners and adapt them to your airgun to shoot slugs, or buy specific, unchocked barrels that have ideal, faster twist rates for heavier projectiles, since pellets generally do best with slow twists, and slugs are far more stable with faster twist rates.
There are formulas to determine the best twist rate for a projectile. It's best to build from the ground up opposed from the up to the ground. Tossing in random projectiles into fx liners is guesswork, which is one approach but its not surefire. You need to slug your barrel, ensure its not choked, and that you can get ideal velocity with the intended projectile weight. Determine the twist rate of the barrel and do some calculations to obtain your ideal projectile weight range.
Shooting slugs takes a lot of more prep work than pellets, that is for sure, even pellets take some tinkering to find out which shoots best out of each gun, slugs are 2-3x the effort.
-Matt