Sizing Cases to Match Headspace
Date: May 04 2010
With nonmagnum, bottleneck rifle cartridges, chamber headspace is measured from the base of the cartridge to the datum point on the shoulder. Knowing the headspace of your rifle and sizing cases accordingly help obtain the best accuracy from your rifle, prolong case life and help avoid case separation.
The best way I’ve found to sort this out is by using Precision Mics (micrometers) from RCBS. They come in pairs. One helps determine headspace and the other helps determine overall cartridge length. The headspace gauge is simple to use and so is adjusting the sizing die based on your headspace measurement. Here’s how.
Select five cases that have been fired in your rifle. They have to be from your rifle, because all chambers are not the same. Measure each case with the headspace gauge. This is done by dropping the case into the base of the mic. It’s the one with the linear marks on its side. Then screw on the other end of the mic until it stops. Don’t force it, just come to a stop.
The linear mark with the zero beside it on the base mic indicates the minimum SAAMI headspace. For my New Ultra Light Arms Model 20 .243 Winchester, I measured five cases and they all measured at -0- on the base mic and plus one on the mic with the marks around its circumference. This means the headspace in my rifle was .001 inch over the SAAMI minimum of 1.6300 inches. When you do this, take the average of the measurements from all five cases.
The next step was to adjust the sizing die so it set the shoulder back between .001 and .002 inch. First, I lubed and sized two cases with the sizing die adjusted per the instructions that came with the die. When I checked these cases with the RCBS Precision Mic, the reading was -0- minus one. This means the sizing die set the shoulder back .002 inch on my cases, or .001 inch under the SAAMI minimum chamber size. For the .243 Winchester this would be 1.629 inch. This is okay because it is the case measurement, not the headspace measurement.