I used to shoot the exact same rifle and I struggled finding the right combinations. When I did though, it was an AMAZING shooter.
For me, I shot loose Pyrodex RS and 535 grain No Excuses conicals the best. My barrel really struggled with 300ish grained sabot bullets, but did really well with 250-260 grain bullets in the red sabots. I took muleys with 260 Nosler Partitions, 250 grain Nosler Hot shots, 430 TC Maxiball and the 535 No Excuses conical.
My brother still shoots the same rifle and has found good enough accuracy with the Powerbelts. I was unimpressed with how they worked on his small forkhorn, but it was still dead. He was shooting the all lead bullet that weighed around 350 grains if I recall correctly.
I found the #11 Mag caps were the best for me too.
Good luck!
FH