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Why check headspace on an AR?

I have a Krieger barrelled AR in 223, and have measured around 0.002 setback from a fired to unfired round. The wisdom is that we should not shoot if the bolt closes on the no go gauge, but isn't this what we're doing when we fire form cases anyway?
 
Why would you fireform in an ar when almost everyone I know full length sizes in an ar-15 for positive function.Am I missing something here? If it closes on a no go gage then you should have someone look at the barrel extension.Was the extension headspaced with your current bolt?
 
I believe that if the bolt closes on the no-go gauge then the case will experience excess stretching and may have a case head separation or if not uniform in thickness may get pierced at the thin point.
 
I phrased the question poorly:

1. If I know that my fired cases are only 0.002 longer than an unfired, why bother checking headspace?

2. Some people routinely shoot with excessive headspace to fire form brass without problem, so what's the big deal about too much headspace?
 
Viperdoc

Excellent points. Some thoughts here:

1. The reality is, if you are measuring your shoulder bump on re-sizing, and you are careful not to work the brass excessively, there is typically little need to repeatedly check your rifle's head space, although checking every 1000 or so rounds is not a bad thing. In and AR-15 (depending on the use and lubrication used) the head space gradually does increase with use (i.e. AR bolts and the barrel extensions are subject to "severe duty" and experience wear due to use, since the bolt is forced to open while there is considerable pressure still in the chamber). The potential head space issue also relates to how far in factory type loaded ammo will go into the chamber without being held back by anything and whether it can potentially fall into the chamber too far so that it potentially leads to marginal or inadequate ignition or delayed ignition, and thereby potentially causing function or safety issues (in addition to potential case head separation where the case enters the chamber too deep, gets fired and stuck deep in and the stretching of the case happens at the back near the web instead of at the front at the shoulder).

2. Bumping the head space of fired cases only .002" for an AR is generally considered "on the edge" to guarantee reliable re-chambering because cases in an AR are extracted while under pressure and they can swell larger than the chamber they came out of. Generally .003" - .005" is typically the safe number to guarantee 100% chambering.

3. For the 6mmAR Turbo 40 we do fire form cases in an AR, and you are correct that we are blowing the shoulder way forward, and the head space of the chamber is way greater than the initial ammo we put into the chamber. The difference is when we fire form, we set the fire form cartridge up so that it blows the shoulder forward (and does not stretch the area forward of the web of the case). That is why on fire forming people use different methods to hold the case back (false shoulder method or rounds with bullets loaded well into the lands with strong neck tension).

Robert Whitley
www.6mmAR.com
 
+1 for Roberts comments that is why I asked him to explain.Thanks Robert as most of your comments come from your vast experience and appreciated by all.
 
Viperdoc,

Robert's comments are spot on. The first thing to understand with an AR (or any Service Rifle, or autoloader for that matter), is that it's not a bolt gun; they're different, and an entirely different set of rules apply to them. Some of the techniques that are commonplace, even beneficial for a bolt gun, are downright troublesome or dangerous in an autoloader. Forget the .001"-.002" world of boltgun headspace and go with the .003" (minimum) to .005" range he suggested. For the Garands, if I recall correctly, the arsenal spec was close to .006". Just remember, all these (US) martial arms have floating firing pins, and minimum headspace increase the potential for a slamfire.
 

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