rickyp, first of all, I've got a lot of experience messing with this .454 12" barrel. I have had very, very little success with getting a 300 gr. cast GC to group with anywhere approaching max loads, which you're getting close to. I've had great success with heavier bullets and heavy charges of H110/W296, and good success with 300 gr. jacketed bullets and 30+ gr. of H110/W296. And before you give up, try some milder loads and see if they'll group, mine really like 14 gr. Unique behind that 300 gr. GC. Don't worry about sighting in until you have a good group to adjust from. Another thing that I really enjoy about the .454 is the versatility, mild loads are great for lots of practice and shoot very, very well out of my barrel, 1 qt. oil bottles are rarely missed at 100 yds. with the 14 gr. Unique combo, 320 gr. LFN GC and 27 gr. H110 is deadly at that range and target as well, I've even regularly hit 1 gal. antifreeze bottles at 150yds.+ with the 320gr. load. Have fun, don't despair, and maybe your scope is giving you fits! I shoot mine with a 4x Leupold EER, sighted it in about 3 years ago, haven't touched it since.
selmer