da new guy-
I haven't shot the285gr variety (only the 245 gr spitfire). The diameter is 0.451 for both projectiles. I asssume that the sabots and bullet profile is the same in both. When I first bought my omega I gave up on loading them. I just couldn't get them down the barrel. I ended up trying different sabots (crushed rib & other MMP sabots) and they loaded fine but things went way south on the accuracy. I couldn't even hit the paper consistently at 50 yards. Spitfires have a boat-tail and the ribs for the enclosed yellow sabots are not cut to the base. Essentially, the boat-tail is protected in the base of the sabot. This may not hold true for you but I'm very skeptical that a sabot not designed for a boat-tailed bullet will group well. Fortunately, with more shooting and some barrel cleaning with JB bore polish they load easier now. I took a deer this year and it put two massive holes in it's chest.
What also groups well in my omega is a crushed rib sabot with a 300 gr speer gold dot or hornady XTP if your father is looking for alternatives. I know many folks have had good results with the speer gold dot bullet. SST's are another good option.
Good luck and please report back if you do try some different sabots and get the barnes bullets to group. Tom