I already have a few of the inserts.See my post above, they used the 38 super mags, but could only get 4-5 rounds in them. The idea died out because nobody wanted a 5 shot conversion gun. The loaded round is about 1/10 inch longer than both the 38 super and the 45 acp. Probably would work on one of those Coonan 357s.
You can also buy chamber inserts to shoot that round in the 7.62 x 54. View attachment 168905 Gun Parts Corp