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It is nice to have the "Widget" made at the time of barreling but, not critical. If you have a cartridge with a 30 degree shoulder (or any other angle for that matter) you can set the compound at that angle and make a "Widget". It doesn't require a reamer.
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DocEd's got the right idea! I hold the size on mine so that when I measure a case I subtract 1" from the reading, it gives me the actual headspace measurement. I've made one for every cartridge I reload.