Mine shoots the 348gr Powerbelt Aero with 90gr and 100gr 777 ffg very well. Looking back on my notes on 10/15/03 it shot groups of 1.2, 1.3, 1.7, 1.6, and 1.3 inches with the 348gr PB.
On 10/18/03 I tried the PR Extreme 300gr for groups of 1.4, 1.05, and 0.8 inches with 100gr 777 ffg. I never felt the need to record groups size with this load any longer. It just kept on grouping.
Those two projectiles were by far the best shooting.
As far as sabots go, I only used supplied sabots/bullets packaged together with that rifle. I believe the Extremes(seat pretty easily) used the MMP high pressure sabot.
Glad you found a solution to your breech plug issues!!