Optima Pro Question

[Ridge Runner]

I bought one last year before hunting season, I've never gotten it to shoot anything near acceptable hunting groups, I've tried every type of bullet I could find with every type of pyrodex made, 5-6" groups was the best I could ever get with magnum loads.
Cut the charge back to 80-90 grains and it gets better but thats not why I bought a 45 cal magnum mzzldr.
I've tried
shockwaves
SST's
powerbelts ( drill holes in these and use for fishing sinkers)
hornady sabots with both 357 and 40 cal bullets
great plains bullets
buffalo bullets
my bro in law has a 45 cal encore that has the same problem, think its the twist or bore diameter.
RR