Big Red here with a question from "Don Adams"...
So, TAKE FIVE!
Big Red, thanks for the Chiefs “low dome” answer, but you...
"Where do collectors come up with post war helmets being a ½-inch smaller draw than WWII helmets?"
Great to hear from you Don,
Answer: The “Grandma’s Yardstick” method of measuring depth.
Most individuals attempting to determine the difference in the depth of draw of M-1 helmets from different eras usually arrive at their dimensions by placing two early McCord Radiator helmets roughly three feet apart on a tabletop between which they balance Grandma’s wooden yard stick.
Helmets of different eras are then placed under the stick between the two McCord helmets and out comes the tape measure. Measuring from the top of the table to the bottom of the yard stick and voila! a ½” difference. The danger of measuring in this way is that it allows chin strap loops and the severity of the visor spanking operation to weigh in on the final measurements.
The actual depth of draw is from the furthest part of the dome, roughly between ballistic zone 6 and 7, and the edge of the rim where the chin strap loops are affixed, plus about a ¼-inch to adjust for the outward flare of the rims gutter.
Unfortunately, the location of the loops and their relation to the varying severity of the down angle of the visor prevent an accurate measurement when helmets are placed on a flat surface. As a collector you have probably noticed that some loops extend past the edge of the helmet body while some do not and the severity of the downward visor spank of an early McCord as compared to the minimal visor spanking of a late war Schlueter is considerable.
Determining the depth of draw is not the same measurement as how high a helmet measures when resting on a tabletop. Using the “Grandma’s Yardstick” method to compare the depth of draw even with two early McCord helmets, knowing both were produced with the same initial 7-inch deep draw, will result in two different depth measurements.
In 1944, The Ordnance Department conducted tests and determined that a depth of draw of 6 ¾-inch was the absolute minimum that would still allow for the proper fit of the existing plastic liner. The fact that a WWII manufactured liner fits correctly into every production model of the M-1 helmet from its birth in 1941 to its cancellation in 1998 indicates that the depth of draw was never reduced more than ¼ of an inch regardless of how they measure up on the dining room table.