I bought a 1:24 twist years ago, wanted to try a conical to cast. Unknowingly, I purchased the 250 grain Lee REAL mold. It didn't work for squat. That tight twist demands a heavy, 360 grain improved Minie or heavier or a sabot. The 250 REAL MIGHT work in a 1:66, but that's really a round ball twist. I'm thinking 1:48 might be the target twist rate for that bullet. .50 cal BTW. If I was shooting a 1:66, I'd stick to patched round ball. I'm kind of wanting one, actually, to shoot patched round ball. With conicals, you have to scrub the bore at least every 3 rounds to maintain any sort of accuracy. The patch will scrub the bore for you and you can get more shots per scrubbing. Scrubbing the bore at the range is a royal pain.