Quick background...been building and chambering all my rifles for25+ yrs. Primarily a HP/SR XC & LR shooter and have done very well over the years. I have always used complete sets of HS gauges when chambering and building. No issue there. For simplicity sake - let me limit this discussion to .308 Win. I normally chamber/ HS to 1.631" in a bolt gun and 1.634" in a gas gun. Staright forward stuff. What starting me thinking and ended up resulting in insomnia was the transferability of measurements between gauges, measuring tools, dies, brass and everything else connected to HS. I checked the HS of a 40X .308 rifle with Forster HS gauges recently - 1.631" - right where it was supposed to be. I moved to check the brass that I thought I had fired in this rifle - not having a baseline and using a Stoney Point (Hornandy) insert that was stamped .400" (datum line for the .308) I decided to "calibrate" the insert gauge on the actual Headspace gauge.....WOW. I zeroed the dial calipers (8" Mitutoyo Digitals to .0005), inserted the HS gauge and I got a HS reading that was .011" short of what the gauge was stamped. (1.630") I checked every gauge - each one was .011 short. This lead me to check the Stoney Point insert - the diameter of that hole (datum line) should be .400" - it was...so where is this massive error coming from? I decided to check a different method - using a Redding Instant Indicator set up for HS and the proper .400 Redding insert - I measured all the gauges again - they were correctly sized relative to one another; .001 steps from 1.630 - 1.638. This set up will not give a direct reading - just a comparative one. So - I measure the brass with the Stoney Point - 1.631". I put the Forster HS Gauges back in the caliper / Stoney Point - .011" short. Check the brass in the Redding Instant Indicator: +.001" right on. So, now I am wondering - what is going on? What am I missing? Any understand this or have experience on this? I get there is tolerance stacking and variability between mfr's - but this is just down right bizarre. Is it related to the shoulder angles of the gauge or the contact edge of the Stoney Point insert? I have considered making my own inserts gauged exactly to datum line dimensions. Any help would be appreciated. How can this be? What am I missing?