260 Headspace

kyshootnman

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Since RCBS does not make a precision mic for 260, it got me wondering. Is the reason because the headspace of the .308 family of cases are identical? I realize there will be some variance, but looking at the dimensions, there isn't much. If I took an initial reading on new unfired brass, then took readings of fired brass, would the .308 Mic be practical? I already own the Mic so giving it a shot would cost nothing.
 

Fuzzball

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"If I took an initial reading on new unfired brass, then took readings of fired brass, would the .308 Mic be practical?"

Partly. The reading of fired and resized cases apply but the unfired/new brass dimension is irrelivant to what a reloader needs to know. The same shoulder tool will do fine for .243, .260, 7-08, .308 and .358.
 

pjparker

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+ 1 for Fuzzball's comment above. I would also submit that the headspace of all of the .308 family is also irrelevant. What is relevant is the headspace in your rifle/chamber/brass combination and not what some spec sheet says it ought to be. The .308 Mic you have should work just fine in .260, but understand that the only numbers that matter are the ones that pertain to YOUR rifle's chamber, and the ammo you load to go into THAT rifle.


edit : To expect that the Mic will read zero for you on your fired brass may be too much to ask. I don't know it you can adjust it to read zero for you or not.

Paul
 
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kyshootnman

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+ 1 for Fuzzball's comment above. I would also submit that the headspace of all of the .308 family is also irrelevant. What is relevant is the headspace in your rifle/chamber/brass combination and not what some spec sheet says it ought to be. The .308 Mic you have should work just fine in .260, but understand that the only numbers that matter are the ones that pertain to YOUR rifle's chamber, and the ammo you load to go into THAT rifle.


edit : To expect that the Mic will read zero for you on your fired brass may be too much to ask. I don't know it you can adjust it to read zero for you or not.

Paul
I respectfully disagree. The .308 family of cases headspace is very relevant. Headspace is measured off a datum on the shoulder therefore would you use a 223 mic to measure, absolutely not. I wasn't expecting the reading of my chamber's headspace to be 0, but I'd like to have some sort of benchmark to reference my chamber's reading to. That's why I'm going to measure an unfired case to note the growth. You can't adjust a RCBS mic. You turn until snug on the case and get a reading. Thanks for your input.
 
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oldfatguy

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Since you already have the 308 gauge no reason not to use it to find out where the shoulder datum is for your chamber. You probably already figured out that you will need something other then the RCBS gauge to measure the throat.

OFG
 

kyshootnman

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Since you already have the 308 gauge no reason not to use it to find out where the shoulder datum is for your chamber. You probably already figured out that you will need something other then the RCBS gauge to measure the throat.

OFG

Yea I made an OAL gage using a fired case from each of my chambers and threaded the flash hole end of the brass. Screw all-thread in with bullet loose in neck until the bullet hits the lands. Works great.
 

Fuzzball

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"I respectfully disagree. The .308 family of cases headspace is very relevant. Headspace is measured off a datum on the shoulder therefore would you use a 223 mic to measure, absolutely not."

One, you were specifically addressing the RCBS "head space" gage (it's actually a case gage; headspace is in the chamber) of the .308 family so I didn't think it necessary to warn you of the impossiblity of trying a .223 or .30-06 RCBS Case Mic to check your .260 ammo. (Seems I was wrong tho!)

Two, new ammo isn't made to be a headpace gage so you certainly won't get a 'zero' on much or any of it. New cases are always made slightly smaller than minimum cartridge size to insure it wiill chamber in anything so no matter what number you get on new stuff it will be meaningless in application.

Three, the datum line is meaningless to anyone but a chamber or die cutter.

Four, all of the .308 family use a .40" datum so it won't matter what RCBS Case Mic thimble is used for any of them. Fact is, reloaders only need a repeatable point anywhere on the case's shoulder to serve as a comparitive (not absolute) reference so he can properly adjust his FL sizer die. Thus, actual headspace, as such, is irrelivant to a competent handloader; ditto an incompletent one for that matter! :)
 

msiebert

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Well yeah, I agree with alot of the above but I always like to know exactly what my rifles headspace measures so I can adjust my dies accordingly to a known reference. I also have a know factual measurement so i can monitor if the rifles headspace is changing...{wear on the lugs, etc.} Admittedly it wont change much on a 260 but some of the bigger magnums...

The datum line per se' is not meaningless, as that is where at least one point of the measurement {actual or just a comparison point} is being taken. The diameter of the shoulder or point that is being used as a datum line dont really matter if done for comparison purposes as suggested, I think that may be what you mean when you call the datum line meaningless. Going by case stretch is not as constant as simply subtracting from actual known headspace. Case stretch can be variable and differ among different ammo, loads, chamber finish, brass case alloy, etc. Simply firing a case and sizing to say .002" less just dont seem to be as accurate as knowing you have a 1.630" headspace chamber and sizing to 1.628"
 

kyshootnman

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"I respectfully disagree. The .308 family of cases headspace is very relevant. Headspace is measured off a datum on the shoulder therefore would you use a 223 mic to measure, absolutely not."

One, you were specifically addressing the RCBS "head space" gage (it's actually a case gage; headspace is in the chamber) of the .308 family so I didn't think it necessary to warn you of the impossiblity of trying a .223 or .30-06 RCBS Case Mic to check your .260 ammo. (Seems I was wrong tho!)

Two, new ammo isn't made to be a headpace gage so you certainly won't get a 'zero' on much or any of it. New cases are always made slightly smaller than minimum cartridge size to insure it wiill chamber in anything so no matter what number you get on new stuff it will be meaningless in application.

Three, the datum line is meaningless to anyone but a chamber or die cutter.

Four, all of the .308 family use a .40" datum so it won't matter what RCBS Case Mic thimble is used for any of them. Fact is, reloaders only need a repeatable point anywhere on the case's shoulder to serve as a comparitive (not absolute) reference so he can properly adjust his FL sizer die. Thus, actual headspace, as such, is irrelivant to a competent handloader; ditto an incompletent one for that matter! :)

Thanks so much for your contribution!
 

Fuzzball

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"Simply firing a case and sizing to say .002" less just dont seem to be as accurate as knowing you have a 1.630headspace chamber and sizing to 1.628"

Loading is about specific effects, not mental self images. No matter how you may feel about the way it's expressed, what you end up with in you're example is 2 thou of change so there's no practical difference and it wouldn't matter a bit where on the shoulder's flat you measured from - so long as you're consistant of course

As practical matter, it's much more difficult to measure headspace (which IS the chamber, not the case) to a thousanth than you seem to understand - so we work with a case, which is, after all, the only thing we can change.


KY - You're welcome, anytime. But then ... what would be the point? :)
 
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kyshootnman

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"Simply firing a case and sizing to say .002" less just dont seem to be as accurate as knowing you have a 1.630" headspace chamber and sizing to 1.628"

Loading is about specific effects, not mental self images. No matter how anyone may feel about the way it's expressed, what you end up with in youre example is 2 thou of change so there's no practical difference and it wouldn't matter a bit where on the shoulder you measured.


KY - You're welcome, anytime. But then,... what would be the point? :)

Entertainment? :D
 

pawprint2

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Once fired cases DO NOT show any true numbers. If you don't believe this, neck size only a once fire piece of brass, put it in the same chamber (oriented the same if possible) and fire it again, then again measure. You may have to do this 3 times, there is spring back in brass, if you are going to be "bumping"0.002, bumping from what? Brass that does not actually show chamber size? This would be the case in a new once fired piece of brass. I agree with those that point out the SAAMI datum line is not your chamber. I assume what you are trying to do is determine how much "bump" you need so you can set your dies. Fire the same piece of brass 3 times, measure each time, quit when it stops changing size, you'll have it as close as you need to determine you "true" head space.
 

sentry1

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Paws makes sense. So... fire, measure, neck size only, reload, repeat, repeat until there's no change in shoulder length?

Once fired cases DO NOT show any true numbers. If you don't believe this, neck size only a once fire piece of brass, put it in the same chamber (oriented the same if possible) and fire it again, then again measure. You may have to do this 3 times, there is spring back in brass, if you are going to be "bumping"0.002, bumping from what? Brass that does not actually show chamber size? This would be the case in a new once fired piece of brass. I agree with those that point out the SAAMI datum line is not your chamber. I assume what you are trying to do is determine how much "bump" you need so you can set your dies. Fire the same piece of brass 3 times, measure each time, quit when it stops changing size, you'll have it as close as you need to determine you "true" head space.
 

Fuzzball

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"I assume what you are trying to do is determine how much "bump" you need so you can set your dies."

Paw, you are the first I've ever seen who recognises that fired case shoulder (nor anything else) position is NOT chamber size, it has sprung back a thou or two in every dimension, but even that varies individually by the alloy and each case's hardness. That's why I suggest everyone with bolt rifles forget "bumping" a specific or, heaven forbid, some average amount and only restore their longest resized shoulders to match their longest fired shoulders. The resulting ammo will easily chamber but case stretching will be minimised, which is the best we can hope for. I find it works for autos and lever rifles too but saying that out loud brings howls of outrage from SB die owners so I keep it to myself! ;)
 

pawprint2

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Thanks Fuzzball, I agree with you 100%. I was trying to make a point, that seems to have gotten lost on Sentry 1, sorry, I just didn't want to go on and on. Fuzzball you are quite correct regarding the "spring back". And it differs from case maker to case maker, Norma appears to one of the "softer" bands. Another thing that I may not have been clear on (Fuzzball got it), is no chamber is perfectly round, nor the wall thickness throughout the case, therefore: if you are trying to determine the chamber's spec on your rifle you would really need to orient the brass in the same position each time it is fired (not an uncommon practice). I will disagree with you on the idea of forgetting bumping, in some benchrest type rifles it has proven to work, but as this is a tactical type shooting board, your advice is spot on. Making 'really good' ammo takes care, but let's don't start adding steps, that in reality don't change the point standings.
 
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Clark

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This is like "Farmer takes the wife. Wife takes the child"

I headspace my 260 chamber on the lathe using my .308Win Go gauge, my 260 Reamer, the rifle receiver, and the bolt.
Then I have a rifle I have chambered.

Then I shoot brass in the rifle.
How I adjust the sizer die to resize the brass is based on how the brass coming out of the die fits back into the chamber.
I want to push the shoulder back .001", so the brass will easily fit back in the chamber, and yet does not lengthen and/or work harden the brass too much.