6x6,
In the 13+ years I have been shooting smokeless muzzleloaders, I have never been able to get .429 diameter bullets to shoot well at all. The .4515 and .452 diameter bullets have been by far the best shooters. The only thing about AA2015 powder, is that even with a 59gr charge you still are only around 2,000 FPS, if that.
You said that you had some IMR-4227, you may want to give that a try, with charges from 44-48 grains. With 250gr bullets you should be in the 2,250-2,350 FPS range, and with 300gr bullets you should be in the 2,150-2,250 FPS range. I have been shooting IMR-4227 in 10ML's and 10ML-II's for about 6 years now, and it has been a consistant performer, in both velocity and accuracy. One load that I can count on with IMR-4227, is 48gr and a .452 330gr XTP, with MMP sabot. This load is good for 2,220 FPS and 1.25" groups. I have taken at least a couple deer with the load at ranges from 40yds to 250yds.
I am not sure about the 240gr bullets, as I have not shot any. Also, if you can find any IMR-4759 powder, you may want to give that try as well. It has been giving extremely good velocities and accuracy with .451-.452 bullets in the 250gr range.
Also, the 2 sabots that have prerformed the best in the 10ML-II has been the MMP and the Hornday Hi-Pressure sabots.