There's an old story that a Winchester salesman came across an obscure 27 caliber Chinese army round during his travels and this became the inspiration for the 270.
Jim Carmichel theorizes that Winchester wanted a hot new round for their then new model 54 bolt action rifle but didn't go with a 6.5 or 7 millimeter bullet because those were popular in Europe at that time and Americans wanted nothing to do with anything "European" so Winchester split the difference between 26 and 28 caliber.
Who really knows? Probably some dead guy.
