Just a suggestion. You can actually drill a little bit of the back end in until you hit the steel portion and up the expansion factor quite a bit as well. I would check out Lyman data for cast 223 loads, iirc they had a few starting loads that were down around 1000 fps. Lyman makes a steel mold that drops 2 of them with every pour. Big flat nose profile bullet. Did pretty well out of my 9" twist AR platforms at low speeds. I did a similar project with my .30-06 shooting 100 grain half jackets over 5 gr of bullseye. Had to drill the flash holes to 1/8" for uniform velocity but it made a great 50yd grouse load that wouldn't scare off any big game nearby.

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