The .451s with the black high pressure sabots will get you beyond 100 yards. Had the same problems as you with the .429s using 44/50 Speer 270 gr SPs. You can get the .451s in 185 - 230 & 300 grain hollow points. I'm using the 230s now. No flyers like the 44s were giving me fits with.
A good source for them is the link below. Use the "down arrow" on the right side of the page to locate different weights/diameters.
Speer Gold Dot HP .451s.