Tolerances

[billtheshrink]

Not sure how to post a new thread so my questions are many. I am new to rifle reloading, not to pistol. What are the parameters for oal? if 1.760 is the oal. what is the 'wiggle room'? Im told i need not trim brass every times i shoot and all my brass is once fired by me, but some are factory reloads. Any tips? I am looking for a good EASY and fast trimmer/pocket cleaner/ etc. any suggestions? Also i have heard that the case gage is good to let me know if a case needs trimmimg?
Thanks Guys Bill

[Unclenick]

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.

[Jimro]

Im told i need not trim brass every times i shoot and all my brass is once fired by me, but some are factory reloads. Any tips? I am looking for a good EASY and fast trimmer/pocket cleaner/ etc. any suggestions? Also i have heard that the case gage is good to let me know if a case needs trimmimg?

If your brass is between minimum and maximum length after resizing, no need to trim.

Looking for a good EASY fast trimmer? Try Possum Hollow or WFT trimmers, if they make one for your caliber. If not, Lee hand trimmers with the mandrel work really good once you chuck the holder into a drill.

For cleaning primer pockets, stainless steel tumbling media seems to be the way to go. Not cheap on the setup, but I've seen nothing that works as good.

And case gauges are a really good way to know if the case needs trimming. Also a good way to get a good measurement on how much your dies are working the brass.

Jimro

[Nathan]

The only real way to know max is with a chamber cast and some pressure test equipment.

BUT,
No need for all that. Your reloading manual will list a max case length. That is max. It will also list a trim length. This is commonly accepted as the min. There are those who cut shorter, but why??? I don't. Shorter affects your bullets ability to hold the bullet well.

You always need to measure your length after full length sizing. That is when the case is longest. A caliper works fine for this. Be sure to rotate the case to find the max length when measuring. Also, be sure the case head is touching the base of the caliper. Sometimes primer crimp or primer makes this stick up.

[Unclenick]

Most trim-to lengths are actually right in the middle of the tolerance so a manufacturer has maximum room for error either side of it. If you measure new bulk brass you'll find home trimming is usually more precise than factory, so we can cut a little more, as some do in order not to have to do it again so soon, or a little longer, as some do to better grip bullets seated out long for single loading accuracy.

[billtheshrink]

Geez Guys thanks for the information. I learn something new every day! Random 223 brass measured at 1.755 inches, THEN i see that id be better to meaqsure AFTER resizing abd depriming! Thank you guys. Im sure i will have more questions as i continue!

Bill