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sizing help

1685 Views 17 Replies 8 Participants Last post by  amk
From what I've read here I have been resizing incorrectly. My brass comes out of the rifle at 1.640, the bolt closes on a 1.638 case, the builder says its a 1.632 chamber, I've resizing to 1.630 per Zediker, is this why my last 1000 reloads suck?? I need to resize to only 1.638? thanks ahead of time!
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I used to turn off the gas cylinder spindle valve and "fire-size" my match brass. and manually extract it from the chamber after a few seconds of cool-down.

That was in the days when I shot local matches. I would then use the RCBS case measurement gauge to get a numerical idea of my chamber dimensions and adjust my sizing die accordingly. Worked ok for my level of (non) proficiency

Sizing and/or measuring off an automatically-extracted case won't give you an accurate picture of the chamber due to the extremely close timing of the firing-extraction cycle in the M14

since extraction begins before all the gas pressure has left the barrel,,,,the case still has not fully relaxed its grip on the chamber walls........... the result is case stretch when the bolt starts yanking the brass out ...

which results in bad (oversize) readings of the chamber dimensions

all fun stuff....

the only consideration I would have in how you are backing into the the proper chamber sizing is the potential for case neck stretch from the expander ball in the die...

with repeated sizing to get the shoulder right, you could be tugging at the case neck with the expander ball which might give an off-reading.

If the case is trimmed back to 2.005 and you get a good shoulder before it pulls/stretches beyond 2.015 you should be OK.

Also....keep in mind that you have sized and resized one piece of brass down to the dimension you want. The next fresh case you size may not behave the same way when run through the "adjusted" die just once.

If you want to get really deep into this you can have a chamber casting made and have one of the die makers machine a sizing die to your desired/exact dimensions

At the end of the day I simply cannot out-shoot most of the M14 types I have put together...they are always better than I'll ever be so I need to draw the line somewhere...or move over to rail-guns in 6ppc
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Very true that is why fire-forming with the spindle shut to off will help you get a better case to work with, you removed the extraction variable

Yet another item to consider when you are trying to get a reading on any case fired through an M14 clone is that the ejector presses on one side of the case while the bolt-lugs rotate and the fired case begins to extract. When the bolt travels rearward the ejector is still pressing on three sides of of the still pliable just fired case(bolt lugs rotation). The case head/extraction groove isn't the same dimension all the way around to begin with then we combine that with the violent extraction of the burning hot case in an M14 clone and the problem becomes exponential. If you turn the same case 3 times in any tool designed to measure set-back you will get three different readings.
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