Exceeding safe loaded case neck diameter?

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sliceman

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How much chamber clearance around the neck portion of a loaded cartridge is considered safe? In the case of my 7-08, both my commercial (Federal) and reloaded cartridges (Winchester cases) have a outside neck diameter of .310-.311 before firing, and .316 after firing. Allowing .001 springback after firing, this would suggest that my chamber is .317 or so in diameter at the neck.

I have recently resized some Lapua .308 cases to 7-08 and find that without neck turning that the outside neck diameter of a dummy loaded case is .3155
The maximum loaded diameter shown in reloading manual diagrams is .3150
My question is: Do these cases need to be neck turned(which I was hoping to avoid as I do not own the equipment) or can I load and shoot them without turning as many members on this forum seem to do, seeing as I am only .0005 over the maximum diameter?
 
To do this safely, the answer depends on how much work and attention to detail you're interested/able to put into this.

The default answer, suitable for reloaders of all skill levels, is "do not exceed published SAAMI dimensions". And unless you take the time to understand all the whys and wherefores it would be a good idea to stick to this rule.

If you are willing to do the work needed to cover all the bases, then clearance is clearance (which is to say, no matter how little clearance you have; so long as you actually *do* have clearance, it is safe). If you have a good set of metal calipers that can lock in position, or even better a 0-1" micrometer, I think you could safely use this brass in this chamber, provided that you understand and do all of the following:
- don't assume .001" springback. Unless you make a chamber cast and measure the actual chamber diameter, the only thing you can safely assume is that a loaded round neck diameter of .316" or less is safe, and any more is potentially dangerous.
- measure the diameter of fired case neck carefully and accurately. Use a micrometer if at all possible. Measure it all along the neck. Establish as precisely as you can what the fired case neck diameter is (is it .3160"? .3164"? etc)
- set and lock your micrometer to a slightly smaller size (say four of five tenths of a thou smaller). Measure each piece of brass when it is loaded with a bullet, ensure that it fits in your micrometer. Rotate the brass so that you are measuring all around the brass, in case there is a high spot in a certain orientation.
- this will be fairly tedious, but not quite as tedious as actually neck turning. Plus you don't need a neck turning tool.
 
I expanded a fired case from .316 to .320 and was able to chamber it although it was tight. After removing it using a cleaning rod, it measured .3185. I then lightly compressed the neck using a Lee collet die down to .3172 +/- .0007 and at this diameter it chambered with barely any resistance, so my chamber's neck diameter should be in the range of .317 to .3175. This would indicate a clearance of .0015-.002" on the loaded Lapua brass(necked down from .308 to 7mm). Would it be considered safe to fire rounds with this amount of clearance?
 
In my past experience if you dont have enough clearance you will suffer in accuracy.
Many of my guns require fitted necks I try and leave between 2 and 3 thous clearance ,there is also a difference between a concentric neck and a neck that has not been worked with a neck turning tool.

If your necks are only about 1 1/2 to 2 thous clearance from necking down the brass you should skim them with a good neck turning tool to make them concentric.
 
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