i have the same gun and got hornady ammo in 45,454,460 and they all seemed to group fine.
had to adjust aim point for each cal tho.
droped a doe at 80 yd. with 460 last sat with a head shot.---very nice.
i use open sights so maybe the scope is your problem, you might try putting the sights on and trying at 25 yd to see grouping and go from there.