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Head space ???

 
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  #1  
Old 08-21-2011, 10:19 AM
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Head space ???

I've got a couple of questions about measuring head space. Currently I'm reloading for a 30-06 and I'm getting 2 consistant measurements while using the hornday head space gauge. (Bushing C .375) Hopefully this is enough information. If not let me know.

1st measurement is 2.0350
2nd measurement is 2.0380

I have been neck sizing (with a Redding neck sizer die) and half of the brass comes out at 2.0350 and the other half at 2.0380. There are a few stragglers ouside these above and below but the majority fall into these two numbers.

If I full length size a couple they end up at 2.0410 (RCBS die)

All Brass is R&P

Other variables
2 seperate lot numbers of brass

I have not been keeping track of the number of loads on each piece. Each piece has been reloaded 1-4 times

What should I do about this? Do I need to worry about it? How can I get better consistancy with the neck sizing? Do you need any other information?

Thanks for the help.
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  #2  
Old 08-21-2011, 10:39 AM
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Re: Head space ???

First thing you need to realize is that it takes 3 to 4 firings on a case (with normal loads i.e. not low pressure not high pressure) before the case will expand fully to fill the chamber and have a "crush fit" at the shoulder. Here is a typical set of measurements on my 30-06 Steyr

new case - 2.040"
once fired - 2.0485" (then neck sized only with Lee Collet)
twice fired - 2.050" (again only neck sized)
3 times fired - 2.0510" (slight crush fit, neck sized only)
4 times fired - 2.0515" (crush fit, neck sized and size body and bump shoulder with Redding Body Die back to ~2.0510")

So until you have fired each case enough to fully expand the case those case measurements will be in flux.

Second thing to realize is that when you size the case body at all the case body will squeeze like a baloon and push the shoulder forward. The only thing that will prevent the shoulder from moving forward is if you have the FL die or body die adjusted down far enough to start pushing the shoulder back.

So the ideal sizing is to neck size until you get the crush fit and then push the shoulder back .001". Do this on every subsequent firing/sizing and then you have a case that is as close to exactly the same dimensions everytime as you can get it.

Then you just need to regulate your annealing so your bullet grip will be as consistant as possible.
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Old 08-21-2011, 05:23 PM
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Re: Head space ???

+1 Woods
He nailed it.
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Old 08-22-2011, 10:23 AM
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Re: Head space ???

I did not know that. Thanks for the information. I haven't annealed yet, so I'll need to figure that out next.

How much of a difference do my head space numbers make? With the way this rifle is set up I'll only use it to 300 yds at most. With .003 difference how much of an issue will that be at 200-300 yds?
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Old 08-22-2011, 05:07 PM
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Re: Head space ???

There are so many variables that you may not be able to tie any effects directly to a headspace difference of .003". Probably not enough to matter or be discernible.

IMO you will get more variance from a seating depth change of .010" from one shot to the other which will effect your velocity and thus your POI and group size.

I will postulate this, if your rifle is shooting 1" or less at 100 yds or 2" or less at 200 yards, there are not any small variations in case prep that will throw you out of a kill zone at 200 to 300 yards. IOW your group size may go to 5" at 300 yards if your velocity drops out of or climbs out of an accuracy node because of small variations, but that ain't a lost big game animal (same bullet, same charge weight, same primer, same powder not changed). If you are shooting a 3" group at 300 yards then a headspace difference of .003" will probably not make any difference.

But if you want to shoot a 3/4" group at 300 yards and not have it be a rare occurence, then you need to control all the variables.
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Old 08-23-2011, 08:35 AM
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Re: Head space ???

Woods,

Thanks for the help.
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