Well, when you check the drawings from SAAMI and the Traditional drawings for the Ackley Improved you find that indeed there is a numerical difference between the two drawings of .014″. The problem stems from reamer makers applying a datum line to the Traditional Ackley drawing at the .375″ diameter on the shoulder, this is not where the datum line is on a traditional Ackley.
The drawing calls for a 40 degree shoulder, but the datum measurement is based on the traditional 17 degree gauge. In other words the systems of measuring are mixed. Ackley used the junction of the neck and shoulder to headspace his Improved chamber, not the datum line along the shoulder.
Hence we end up with a headspace length of 2.1542″ on the Traditional drawing. I measured this length on my cutaway chamber and guess what, that is the length to the junction of the neck and shoulder. Thus headspace matches the drawings correctly.
The 40 degree SAAMI gauges are made to the industry standard, datum line method. So the drawings are correct, the datum line is at the .375″ diameter along the shoulder. The length called out for this gauge is 2.140″, which appears to be .014″ shorter than the Traditional design.
What we have is two different methods of measurement. However they achieve the exact same result.
