ABOUT CARTRIDGE CASES
This summarizes the results of tests and measurements performed in 2012.
For detailed test results and measurements see 6.2 CASE SHOULDER SETBACK WITH REDUCED LOADS and/or 6.2 CARTRIDGE HEADSPACE AND LENGTH CHANGES in CAST BULLETS FOR BEGINNER AND EXPERT, THIRD EDITION.
Definitions of these words as used below are from the SAAMI glossary: HEAD CLEARANCE The distance between the head of a fully seated cartridge or shell and the face of the breech bolt when the action is in the closed position. Commonly confused with headspace. HEADSPACE The distance from the face of the closed breech of a firearm to the surface in the chamber on which the cartridge case seats.
Thanks to Kent Sakamoto at ATK/RCBS for lending me the RCBS “PRECISION MIC” headspace measuring micrometers used in these experiments.
CARTRIDGE CASES DO GET SHORTER WHEN THEY ARE FIRED; AND LONGER WHEN THEY ARE FULL LENGTH SIZED. As much as .007” for .223 Rem. and .243 Win. cases, and as much as .013” for .308 Win cases.
PULLING THE CARTRIDGE CASE OVER THE EXPANDER BUTTON IS NOT THE CAUSE OF CASES GETTING LONGER WHEN THEY ARE FULL LENGTH SIZED.
CASE HEADSPACE IS REDUCED AS FIRED CARTRIDGE CASES ARE FULL LENGTH SIZED. As much as .008” depending on chamber headspace and sizing die adjustment.
SIZED CARTRIDGE CASE LENGTH VARIES SLIGHTLY AS THE FULL LENGTH DIE SETTING VARIES. Die setting instructions for FL sizing vary by manufacturer and in the literature. Maximum of .002” for .223 Rem. and .308 Win. cases.
CASE HEADSPACE VARIES AS THE FULL LENGTH SIZING DIE SETTING VARIES. As much as .004” in .223 Rem., and .008” in .308 Win.
NECK SIZING IN A FULL LENGTH SIZING DIE DOES NOT CHANGE CASE LENGTH.
NECK SIZING IN A FULL LENGTH SIZING DIE INCREASES CASE HEADSPACE. Depending on die setting etc., as much as .005” in .308 Win. cases. Could this have to do with “bumping back” case shoulders?
EXPANDING/BELLING CASE NECKS WITH THE LYMAN “M” DIE DOES NOT CHANGE CASE LENGTH OR CASE HEADSPACE.
SIZING IN THE LEE COLLET DIE DOES NOT CHANGE CASE LENGTH OR CASE HEADSPACE.
REPEATEDLY SIZING IN THE LEE COLLET DIE DOES NOT CHANGE CASE HEADSPACE.
NECK SIZING IN A LEE LOADER DOES NOT CHANGE CASE HEADSPACE.
THE PUSH FEED EXTRACTOR DOES NOT CHANGE CASE HEADSPACE.
FIRING VERY LOW VELOCITY LOADS, (VLVL), DOES SET THE SHOULDER BACK, DECREASING HEADSPACE.
SHOULDER SET BACK WITH VLVL IS CAUSED BY THE PRIMER FIRING.
SHOULDER SET BACK WITH VLVL IS NOT CAUSED BY THE FIRING PIN HITTING THE PRIMER.
SHOULDER SETBACK WITH VLVL IS NOT CAUSED BY THE PUSH-FEED EXTRACTOR.
THE EXTRACTOR DOES LIMIT SHOULDER SET BACK WITH VLVL.
SHOULDER SETBACK WITH VLVL CAN BE ELIMINATED BY OILING THE CASES BEFORE FIRING.