If I were going to try that, I'd opt for the .22 TCM case over the 5.7 just because the 5.7 case is a lot more fragile. Necking down a .22 TCM case to .20 would be easy enough. Making cases, although tedious, would be relatively easy. Getting a reamer and dies might be the hard part, all to gain what? Performance would probably be similar to a .17 Hornet or .17 AH. Whatever makes ya smile!
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