August 28th, 2006, 11:24 PM
Join Date: Apr 2006
Like an RCBS Precision Mike
Search the internet for an RCBS Precision Mic, the Mo's is a forerunner to that, and a scarce little jewel, to boot. I have a buddy that has them in 308 and 3006. I would think that they most likely date from about 20+ years ago.
The Mo's gauge, and the RCBS Precision Mic, measure the length of your cartridge case to a specific datum line on the case shoulder, and will tell you the relative length of the cartridge. For example, an unfired 308 Win cartridge manufactured to fit a 1.630 (tight 308) chamber might read -5 on the gauge. This means the cartridge is .005" shorter than the chamber. Now, fire that cartridge in that chamber, and it should read fairly close to 0. This would correspond to 1.630.
Take that same -5 308 Win, and shoot it in a 7.62 chamber headspaced at 1.634, and the fired case should (theoretically) read +4. It would have grown .009".
What these gauges tell you are two things. First, the relative length of an unfired cartridge, and then the increased length of the fired case. With this info, and either minute sizing die adjustment (or shimming) or Redding Competition Shellholders the resized case length can be varied to fit individual chambers. Most match shooter try to fit a resized case to about .002" smaller than their chamber, and this is much more difficult to do without a case micrometer.
It can also be used to spot ammo that is too long to fit properly in a tight chamber. Again, an example: My 7.62 Garand is chambered with a tight (1.630) .308 chamber. Approximately 25% of the South African 7.62 is too long for this chamber, and about 40% of the late Lake City 7.62 surplus is too long. Perhaps I am being too careful, but my thoughts are that ammo too long for my chamber is a potential slamfire, and is a possibliity to be avoided. A few twists of a pull through reamer can also remedy this situation, and the case gauge, along with headspace gauges, will tell you exactly where you are.
One other thought, the precision mic (and the Mo's gauge) should really be checked with a quality headspace gauge, in order to maks sure they read correctly. My RCBS Precision Mic reads -2 on a Clymer .308 Win Go gauge. It should read 0, but by checking it against a known gauge, the relative case and chamber lengths can be easily figured.
Now I bet you know more than you needed to about case length gauges!
Last edited by relics6165; August 29th, 2006 at 08:52 AM.