You have a rifle that has movement, it can be pulled to the rear 'ZERO', it (the bolt) can be pushed forward 'X', with any type of tool that measures height and or depth (laying down would be length) you are half way there to measuring head space, just simply chamber a round that you are going to shoot in the rifle, pull the bolt to the rear, and measure from a flat on the rear of the receiver to a flat on the rear of the bolt for 'ZERO' then push the bolt forward, and measure again, the difference between the two readings equal head space, or you can purchase a head space gage and repeat the procedure, not a problem but you will not be shooting gages.
The case to be fired is actual head space, in the real world the go-gage is .005 longer than the minimum, full length sized, commercial case from the head of the gage to it's shoulder, and you already have the firing pin removed, something I do not find necessary, I use transfer cases, sized cases with seated bullet, no primer/no powder. And on the rifles with bolts that move forward and to the rear like the 03 type rifle, I use the feeler gage for head space in thousands.
Most fire a case to determine the effect the chamber had on the case when fired, I determine the effect the chamber will have on the case when it is fired, I have no problem finding cases that require sizing before chambered, my methods eliminate one trip to the range and I do not shoot gages, I shoot ammo I load.
F. Guffey
