It would not hurt to try 150 grains of pellets, but you might not get the accuracy results you want. If yourwereshooting loose powder I would suggest you kick the load to 110 or 120 grain but your not. As to whether the 300 grain XTP is a 150 uard projectile.. I could not say. I personally would think so, but you would have to try it out on the range. It sounds like you have a 125 yard shooter, so another 25 yards is not all that much further (until you try it).

I have no first hand experience with the killing power of the XTP. I have read a lot of posts by people that shoot them, and they feel they are a real deer killer. Whether the 240 grain would work better or shoot better, only some range time would tell you that. For the long distance stuff you might want to look into a 250 or 300 grain Barnes Expander. They are expensive, but one of the best bullets out there. Also your Knight Disc should love them. I shoot the exact same rifle actually... the older model though with the orange primer holders.