Maximum charges are not always the most accurate, so you're headed in the right direction by changing things up.
Load your rifle with the muzzle UP.
ALWAYS, ALWAYS, ALWAYS...……. YES AGAIN, ALWAYS, USE A WITNESS MARK ON YOUR RAMROD.
Start with the basics, such as are your mounts and rings torqued to specification?
Are you using at minimum a quality scope?
Solid bench and solid rests?

I'm not one to suggest, let alone use powerbelt bullets, but if that's what you're using you might want to consider a much better bullet.
Pellets are not as good (in most rifles) as loose propellant, which can be tweaked. However the right combination when using pellets they will still shoot well.
Pellets will definitely create a situation where you'll need to swab between shots. What you swab with can make a huge difference. For blackpowder or pellets, I highly recommend Butch's Black Powder Bore Shine. One single patch...…. SLIGHTLY DAMP...………. NOT WET...…….. down the bore both sides. Don't use a wet patch. All you need is an ever so slightly damp patch. If you use a wet patch, it can affect the bullet's POI.
Take time between shots to let the barrel cool back down.