I am in utter concordance with this rimfire substitute idea, and have been for a few years. I ended up getting a Lee Bator mold and working up rimfire power level loads for 218 Bee and 223, and assisted my brother in doing the same for his 221 FB, 22 Hornet, 22-250. I opted for Unique and Blue Dot, he decided finally on Titegroup. We both picked two power levels: the lessor at about 1200, using non-GC Bators, and the greater at about 1600 using GC. I'm still working on accuracy problems for power level #3: 2k+. We were using a very soft alloy for the other two, and accuracy falls apart at about 1700 in my bee, and below that in 223. Have some harder bullets ready to load, but haven't gotten to it yet. These cast bullet loads are more accurate than my rimfires.
We moved on to work up loads for 17 Mach 4 and 17 Fireball with 20gr bullets and Titegroup to mimic the 17 M2 and the HMR as well. Accuracy hovers around 3/4" at 100.
