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The best way to do it is to form a "false shoulder" on the case to control headspace when you fire-form. Expand the case necks to .308", then size in the 280 die to .284" only as far back as the end of the neck will be when the case is finished being fire-formed. Annealing the neck after fire-forming will help prolong case life.
Some people have gotten away with just firing the unmodified 270 cases in controlled-feed 280 actions because the case hangs on the extractor near the back of the chamber to be struck by the firing pin. But, in push-fee actions, that my give misfires and will unnecessarily stretch the case if it fires. In push-feed actions, the 270 case would stretch about 0.051" MORE than a factory 280 case stretches when first fired, if the false shoulder is not used. In a controlled-feed action, the amount of stretch depends on how well the extractor holds the case away from the chamber shoulder, if the false shoulder is not first created in the 270 case.
SL1
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