One of the first rules of handloading is to always follow the approved reload data. The cautious reloader gradually works up to approved maximum loads to ensure his particular gun does not show pressure signs. Generally this is visual observation of the fired shell case head and primer. There is another slick way to check for pressure signs if you are interested.
Using a blade micrometer that measures in ten thousandths (.0001″), new, unfired cases can be gauged before and after firing to determine reasonably accurate maximum loads. Micrometers measuring in thousandths (.001″) are insufficiently accurate to perform these measurements, and should not be used. Previously fired cases cannot be used accurately due to various levels of brass hardening. Measurement is taken just ahead of the extractor groove on the case head and must be taken at the same place on the case before and after firing. By placing a small mark on the case head – entering the cartridge in the chamber with mark at 12 o’clock – a consistently accurate measurement can be taken with each firing.
Lower pressure rounds, like the .30-30 Winchester, usually yield maximum pressures at .0003″-.0004″ expansion. Modern cartridges, like the .223 Remington, will show maximum pressure at .0004″-.0005″, while .308 Winchester, .270 Winchester, etc., typically yield .0005″-.0006″ expansion at max pressure. Magnums, like the .300 Winchester Magnum, show maximums at .0006”-.0007” expansion, and should be measured on the belt.
In conjunction with these measurements, case head signs of pressure should be monitored as well. These signs include very flat primers, slightly cratered primers, ejector marks on the case head, and stiff extraction. All these case head signs indicate high pressure, and loads should be reduced until these signs disappear.
As always, start with the beginning load listed, and cautiously work up to the maximum shown for that set of components, using the methods listed herein.