Thread: Tolerances
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Old April 6, 2013, 09:12 AM   #2
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 10,189
Bill,

I made your post a new thread. In the future, just go into the forum in question and click on the New Thread button in the upper left.



Tolerances are unilateral in cartridges and chambers. This is standard engineering practice when an error in one direction has worse consequences than an error in the other direction. In this instance, making a cartridge too big or making a chamber too small jams a gun. That's more of a show stopper than a cartridge that's a little too small or a chamber that's a little too big, which merely result loose fit. So, when you look at SAAMI's drawings, you find the cartridge linear dimensions are all maximum values, with a minus tolerance only, and chamber linear dimensions are all minimum values with a + tolerance only (vice versa for radius numbers, as a little thought will reveal the reason for).

The SAAMI drawings are available here, under Cartridge and Chamber Drawings at the top of the list. Watch for notes on the right side of the page that say things like "all diameters -0.008" except as specified" or other universal tolerances.

Cartridge Overall Length (COL)¹ is given as a maximum than can be expected to fit in a SAAMI compliant magazine and to feed from it. The minimum is also for feed reliability. If you are loading cartridges on-at-a-time, however, or if your magazine is a little extra long or its feed lips and your gun's feed ramp will tolerate a shorter cartridge, then you can use numbers outside the SAAMI limits and chosen to fit your chamber or to use a special bullet you have. Just be aware that changing seating depth from the one listed in a load manual will change pressure. How much depends on the cartridge and chamber. You just have to know how to start with a minimum load and work up from there while watching for pressure signs.

A good rule of thumb in Accurate Powder's FAQ is to find the starting point of your load workup by reducing a rifle maximum load by 10% and a pistol maximum load by 15%. Then follow Richard Lee's practice of making each load a 2% step until you know the maximum shows no pressure signs in your gun. If you get a clear pressure sign, then you back the load down 5% from that load level and treat that 5% reduced load as your gun's personal maximum.


¹ This is also given as COAL, OAL, OL, Cartridge OAL, Cartridge OL, etcetera. The usage of "overall" has changed over time. The word dates back to Chaucer. Originally it only was used to mean "taken altogether", as in "overall, his life was good". When a linear total length was meant, the word was hyphenated to "over-all", as in "the boat is 13 feet over-all". The initial for "overall" is just "o", and the initials for "over-all" are "oa". So the proper name for the length of a cartridge was originally Cartridge Over-All Length, or COAL, as is still used in some references today.

Sometime during the 1950's, though, this changed. If I look in my 1948 copy of Webster's 2nd Edition, the hyphen is there for over-all, meaning a total length. By the time my copy of Webster's 3rd edition was printed in 1962, all the hyphens are gone and "overall" is the only form and is used for both meanings. Thus, you can now use all the forms I showed.
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Last edited by Unclenick; April 6, 2013 at 09:49 AM. Reason: added clarification and information
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