On another forum, someone posted a link to one of the pages of ar15 barrels website, describing a straight walled 5.56 case with a .357 bullet, called .357 Rimless.. and then on a search, found some posts by the guy who owns that website.. turns out the AR magazine design is what is preventing the larger bullets from feedig reliably due to the deep ribs for the bottleneck 5.56 cartridges ... More @Wikipedia
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