I would check out your overall length, if you are loading to the same length as you would for your factory barrel, that could be the cause. Typically the throat and lead on a custom barrel is more precise and MUCH shorter than those in the more recent factory barrels. If your rounds are too long for your throat, your accuracy will go all to heck. I bring this up because you mentioned using the excellent but fairly long 180 grain bullets.
That would be my first test. If that doesn't work I'd recommend trying one of the 4227 powders as they often produce great accuracy, if not top velocities. Gotta admit, I'm with TC Shooter, the H110 or WW296 loads typically are my fastest and most accurate.
If you don't get what you want, either contact Fred at Bullberry and ask about his accuracy warranty or contact me to look into a trade!
regards,
Graycg