Sizing rifle brass to chamber.

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Good question, there’s a key phrase from an earlier post that people sometimes overlook. Chambering freely is the goal so from just smooth bolt closer we bump .002, a lot of guys just bump a fully formed case but we find out that sometimes that’s just not enough for smooth cycling.View attachment 1042117

Why is chambering freely the goal? An inability to close the bolt would be something to avoid, but what is it about smooth cycling that indicates a better fit than a little resistance?


Firing once may not represent the actual dimension of the chamber, so bumping 1-2thou from fired brass may be sizing too much or too little.

Bolt close method can’t lie. The case either fits or it doesn’t, and when it has been sized just to the point of kissing the shoulder, we know a true dimension of the chamber.

Basically, it’s like measuring my sock versus my foot.

I can see how sizing 1 to 2 thou under the dimension of a once-fired case could be needlessly sizing the case down, and the bolt close method would disclose whether the case really needs to be sized down that much or not. Bumping a fired case seems less and less likely to size the case down excessively on subsequent firings. Even so, I can see how the bolt-close method could be used without the necessity of more firings and that it demonstrates a more direct feedback.
 
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To those who might know.......how much potential headspace and "bump" is there is the typical rifle chamber? I was looking at the SAMMI specs for 270 Win......chamber and cartridge......and it looked to me like it would be 0.006" between max cartridge and max chamber.

Or put another way, if the LEE dies (or any other standard set of dies) do what their material says they do......resize back to the maximum allowed by SAMMI specs.....how much is the bump? In theory, a "tight" chamber would be none. But what about a loose or "sloppy" chamber?

There seems to be two issues or methods floating around. Standard dies setup as per instructions........are sizing to a standard. Setting up the dies to close a bolt throws that standard out the window......and sizes to the individual chamber. Just curious what the potential difference between the two might be?

Is it worth chasing......or are we picking fly crap out of the pepper?
I haven’t had my coffee yet so this could be the dumbest post ever or might make sense. The standard difference in length between a go and no go is .004 same as the body reduction of a standard die vs finish reamer, our “match” dies hopefully are minus.002 body reduction. Shoulder bump or creating headspace can be increased or decreased by way of threads, shell holders or milling an extra few thousands off the bottom as with Redding type S (speedy) bushing dies. The gag is who speced the reamer that was used to chamber your barrel and can you get a copy of the reamer print to order a die? If not it’s trial and error with a few options where most guys do the trial and error you read about on line.
Anyway here is a shop tolerance sheet from one of top companies who make reamers.
 
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Why is chambering freely the goal? An inability to close the bolt would be something to avoid, but what is it about smooth cycling that indicates a better fit than a little resistance?




I can see how sizing 1 to 2 thou under the dimension of a once-fired case could be needlessly sizing the case down, and the bolt close method would disclose whether the case really needs to be sized down that much or not. It seems less and less likely to size the case down excessively on subsequent firings. Even so, I can see how the bolt-close method could be used without the necessity of more firings and that it demonstrates a more direct feedback.
(Free chambering means smooth cycling, no one wants to fight a bolt open during hunting season or in battle not to mention during competition.
Galling the bolt lugs can be an additional outcome from excess resistance during bolt manipulation.)
I use a farm boy sketch when setting up dies and measuring body reductions, headspace etc.
 

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Firing once may not represent the actual dimension of the chamber, so bumping 1-2thou from fired brass may be sizing too much or too little.

Bolt close method can’t lie. The case either fits or it doesn’t


True, but I don’t like cutting it that close for anything but Benchrest, and even then I want totally free chambering.

Fair clarification: Bolt lift method just finds the headspace length of the chamber - the loader then has the option of choosing whatever shoulder bump, whatever clearance, they want from there.
 
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