9mm case length

[Kyrie]

Hi Rick,


This actually gets us into the realm of firearm headspace (a subject that starts simple and quickly becomes horrendously complicated) and cartridge length from case head to the headspace datum line.


Generally speaking and all other factors being equal, the more prefect the match between firearm headspace and cartridge length from case head to the headspace datum line the greater accuracy potential the cartridge has.


But to get that perfect match one has to put aside loading manual advice on proper trim length and manufacturer standards for firearm headspace (incidentally, SAAMI 9 mm Luger headspace is 0.754 min and 0.776 max). Rather what one does is to determine the precise headspace of the firearm in question, and then trim (in the instance of a cartridge that takes its headspace from the case mouth, like the 9x19) each cartridge case to be a perfect fit in the specific firearm.


There are any number of ways to go about this. What Iâ??d recommend, if anyone has the time and dedication to get involved, is the following.


First take a chamber cast of the Luger in question. That will give you the length of the chamber from the chamber mouth to the headspace datum line. Add to this the depth of the case head recess in the Lugerâ??s breech block. This will give you a very basic estimate of the headspace of the specific Luger.


Buy 500 - 1000 .38 Super virgin cases. Remove the semi-rim from a .38 Super virgin case. Trim cartridge case to that basic estimate of the headspace plus about 0.01 inch, run it through a 9 mm Luger FL sizing die, bell the case mouth, and run the case into the bullet seating die (no bullet). Take the assembled cannon (only) for the Luger, remove the extractor, and try to chamber this worked cartridge case. If the toggle assembly doesnâ??t go fully into battery (and it probably wonâ??t on the first try), take a fresh .38 Super virgin case cartridge case and do it all over again - but trim an additional 0.005 inch off the case length before running it through the FL sizing die. Repeat as many times as needed until the toggle assembly goes fully into battery. For safetyâ??s sake, deduct 0.001/0.002 inch from this last case length, and you will have a near perfect cartridge case length match to the specific Lugerâ??s headspace.


Now do your load workup, using only brass from the same lot as the original purchased 500 - 1000 .38 Super virgin cases. Use ammunition loaded with these cases *only* in the Luger used to determine their case trim length. If these cases are to be reused, they *must* be trimmed to length before each use. Do not try to run these cartridges through the magazine to the chamber - leave the extractor out of the breech block and hand feed each round (remember - you wanted accuracy, not convenience [smile]). You have just eliminated case length variation from the accuracy equation, and have a very close to perfect match between firearm headspace and cartridge length from case head to the headspace datum line.


The next step in achieving the best possible accuracy involves chamber throat and leade, and cartridge length overall. And then we can talk about sorting cases and bullets to achieve the minimum variation in each. Next up is bullet bearing surface relative to rifling pitch and velocity, And so on - achieving maximum accuracy is a time consuming thing in which to get involved. Only the truly driven become involved


Best regards,


Kyrie

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