English language is funny. It keeps me confused. "bad" was something not good, now I hear it used for something good. Im going on 60 and grew up reloading metallic cartridges. When I was young we had to read the reloading books and learn the terms and definitions. Out of curiosity I drug out some old reloading manuals from the 60's their definitions say a cartridge is the complete unit of ammunition assembled. ie. case, propellant, powder, primer. Slug was defined ie. A cup shaped, hollow based projectile, usually bearing external pre-cut rifling, intended for adapting shotguns to the hunting of large game. Also slug means to force soft lead through the bore of a gun for measurement. Bullet: the missile only, becomes a projectile in flight. Then listed bellow bullet is bullet types hollow point. round nose, Spitzer point. fmj boattail, spire point. wadcutter and semi wadcutter. I have a Master Caster brand cast bullet set up, and have round nose, wadcutter, semi wadcutter and spire point molds for cast bullets.
So why I asked is I see most original airgun slugs are cup based. But some shooters have adapted to use cast bullets and are having success I may add.
Just curiosity my friends.