The .004" you speak of is from the casehead to the shoulder-neck junction. Its the "Correct" way head clearance is established using the parent cartridge in an improved chamber with a .004" crush fit for firing/forming.
There's a conversely if there is a difference between the SAAMI and the traditional Ackley version. Some say they're different, others say they're the same. I don't know one way or the other. My better 280s are the traditional Ackley versions.
http://gunsmithtalk.wordpress.com/20...mproved-alert/
but.....
https://gunsmithtalk.wordpress.com/2...eadspace-test/
...and
http://www.redding-reloading.com/tec...80-changes%29:
Bill
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