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  #1  
Old 02-05-2012, 11:21 AM
mattmattmatt mattmattmatt is online now
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Default Chambering cartridge in AR changes headspace measurement

Using the RCBS precision mic, I measure ~+6 mils on my once fired brass from my AR (BCM, .223 cal).

Just for testing, I resize the brass to +3 mils and make a dummy round (no primer/powder) to check function. Load the cartridge in a mag, charge it, and everything seats without a problem. But when I re-measure the cartridge after the test, it's right near zero.

I didn't expect this, and my only guess is the violence of the chambering is the reason. What else might cause this? And is it worth increasing the headspace if it will just end up back at zero?
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Old 02-05-2012, 3:49 PM
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G-forceJunkie G-forceJunkie is offline
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Not sure where your reading mils, those mics are graduated in thousandths (.001") of an inch. When you say it shortens after you load it, how are you loading? Are you letting the bolt slam home, or are you removing the upper and gentaly sliding the bolt home? Yes, its possable to cram an overlength round into a chamber and make it fit, but its not right. If this is a precision rifle, the only gun this ammo will fired from, and your concerned about brass life, then and only then would I take the time to load ammo that is just sized enough to fit the chamber. Otherwise, just size it all down factory length and be done with it. It will function in any rifle, and function longer as the chamber gets dirty.
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Old 02-05-2012, 4:24 PM
rsrocket1 rsrocket1 is offline
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A mil is 0.001"
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Old 02-06-2012, 2:49 PM
robert101 robert101 is offline
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I don't use "mils" to measure reloading components. A "mil" is typically meant to be a milliradian which equates more to a radius measurement relating to the circumference of a circle. I use inches or millimeters on by calipers. By the way 1" = 25.4 millimeters.
Matt, what is your bullet setback in inches?
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Old 02-06-2012, 3:05 PM
BAGunner BAGunner is offline
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Quote:
Originally Posted by mattmattmatt View Post
Using the RCBS precision mic, I measure ~+6 mils on my once fired brass from my AR (BCM, .223 cal).

Just for testing, I resize the brass to +3 mils and make a dummy round (no primer/powder) to check function. Load the cartridge in a mag, charge it, and everything seats without a problem. But when I re-measure the cartridge after the test, it's right near zero.

I didn't expect this, and my only guess is the violence of the chambering is the reason. What else might cause this? And is it worth increasing the headspace if it will just end up back at zero?
That's interesting. I saw similar thing happening to my brass as well, but not the level of difference you had.

The Mic shows my fired brass at +0.004. After chambering it, it is at +0.003. This is testing with 3 dummy loaded rounds.

Another thing I noticed was the scratching mark on the bullet. Is that due to the feeding ramp? Should I polish the feeding ramp?
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Old 02-06-2012, 4:25 PM
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Quote:
Originally Posted by rsrocket1 View Post
A mil is 0.001"
also a lot easier than saying "three thousandths of an inch"

gotta take the context of the post into consideration...
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Old 02-06-2012, 5:45 PM
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First off, if the chamber is clean and the case shoulder (datum line) has been setback in any manner during chambering from how you set it in re-sizing then you aren't re-sizing the case enuff in the first place.

2nd if you are measuring cartridge OAL (Over All Length) from case head to bullet tip and seeing a reduction in OAL (tip to case head) then this is a separate issue entirely and not a "headspace" or re-sizing related problem.

3rd, if you are not using a comparator gauge to measure shoulder to case head distance of a fired but un-sized case and compare that to a sized case to verify that you have actually set the shoulder back at least .0015" then you don't know anything at all here in the "first" place.

Best gauge for this is the L.E. Wilson comparator one (read the instructions at the link below).

http://lewilson.com/casegage.html

Last edited by GeoffLinder; 02-06-2012 at 5:48 PM..
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Old 02-06-2012, 7:12 PM
mattmattmatt mattmattmatt is online now
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Thanks for all the replies.

All of the measurements I've provided have been in thousands of an inch. I can see how it wasn't clear in my original post.

I just let it slam home when chambering the round. At classes and at the range that's how I typically run the rifle. I have tried easing the bolt forward, and at least for my test cartridges I have to use the foward assist to get it into battery. My headspace measurements are taken with the RCBS precision mic:

http://www.midwayusa.com/product/477...-223-remington

I am 99% certain this tool is providing the headspace measurement, but please correct me if I'm wrong. I use calipers for the overall length, and for the first experimental rounds I've been adjusting the bullet seat depth for an overall length of 2.26".

GeoffLinder raises a good point. The chamber on my rifle probably isn't the cleanest and may be throwing things off. I'll get it spotless and see if anything changes. I'll also measure the bullet setback since I haven't examined that at all.
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