I have noticed that some muzzleloaders with slightly slower twist rates also have a little trouble stabilizing the 300 grain bullets, especially if you use the .44 (.429 actual)bullets/sabots instead of .45 bullets/sabots due to being quite long. But if your muzzleloader stabilizes the 300 grainers, I would definately use them over the 240-260's. Quite a few years ago, I dumped a small muley buck at a walked off 135 yards with my T/C Thunderhawk, and using the two piece T/C sabots and 300 grain 44 caliber Hornady xtp's. The bullet past right through, nice entry and exit wound and the deer only traveled about 20 yards before reaching its final resting place. Can't ask for much more than that in my opinion.