Accuracy can be expected from MOST combinations of quality PCP with quality pellets.
Accuracy from a quality gun with quality slugs is a literal hit or miss proposition.
Of all the slug/barrel combos I've tried, only two were good enough to warrant stocking up on that slug. At last count that was around 40 barrels, various slugs from each barrel. And all the barrels were quality: LW, CZ, TJ, Korean, custom.
In fact, my most recent slug failure was with .22/32.3 Altaros slugs yesterday afternoon. I was in the speed window they supposedly like, from a non choked LW barrel known to be pretty decent with other projectiles. I couldn't keep them on an 8x11 sheet of paper @ 100 yards. Concluded this is another in the "nay" column, retuned for the .22/25.4 JSB MRD and proceeded to shoot an EBR 226 @ 100yards in squirrelly winds. So, good barrel that simply doesn't like that particular slug (a long with many others).