Very good story...but as machinist by trade (even though I no longer work as a machinist) you are using calipers to measure the shell, not micrometers. I hate correcting people but I always feel the need to when it comes to tools.
BTW, this is a micrometer, an outside mic to be precise. The also make inside mics to measure an ID rather than the OD and they also make depth mics for measuring bores,etc.
View attachment 411365
BTW the picture is of a Disk micrometer to be precise!