cost about $5 per box of 5, and are usually of the "Foster" design. Supposedly they don't shoot small groups in a rifled slug barrel, like yours.
The slugs that are designed to shoot well in a rifled slug barrel are usually of the Sabot design. They run about $25 a box.
Then there are the "solid copper" design. Basically the shotgun version of the Barnes "X" bullet, with a soft plastic tip which "sets back" upon impact and causes the 4 copper petals to "open" (expand). Shoots well (small groups) in rifled slug barrels, but way up there in price, too.
So, what do you want - low cost, or better downrange ballistics and better terminal performance? Can't get both with the same ammo.
I'm just saying.