Some very interesting facts! I have been looking at one of my Tantial kits and the 5.45x39 cartridges and I am starting to think that a wildcat in a 300/5.45 might be interesting as well
5.45 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 7.62x39
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I just don't know if I am foolish enough or ready for a project like that anytime soon... I also have not looked at the availability of brass 5.45x39 casings of how similar these would be to 223 brass... Just idle thoughts while looking at a tantial kit I guess :)