Any bullet that will fall through cylinder is undersized for that gun . If it takes more than finger pressure or a pencil to push a bullet through a cylinder it's oversized . Under will lead , over will raise pressure somewhat but better than undersized . Will the slug you pushed through the bore pass through cylinder throats ? If so that's good , if not cylinder throats are undersized & need to be opened up . Also too small a throats will lead every time as bullet has been sized down before it hits the bore . Gas bypass flamecuts bullet base & you got big time leading . The Hornady bullets fall through they're undersize for that gun . If throats are larger than bore a bullet sized to throats with a good lube won't lead . We've all bought bullets that weren't sized as advertized . I'm hoping throats are good & all you need is correctly sized bullets .