Twilight zone headspace?

[BrokenWheel]

I set my headspace measurement to 1.462 on my 223 brass. On my AR it expands to 1.466". The other other day I was shooting subsonic (reduced loads) in this brass (trail boss). Measuring the headspace on these loads showed that the brass headspace had decreased to 1.461.

I double checked and re-zeroed my caliper, even made a small dent on the brass to see if the powder expanded the lower part of the cartridge (it did). despite all this the headspace still seemed to of decreased.

I'm not sure whats going on...

 

[rsrocket1]

Was there a bunch of soot on the neck and shoulder?

It might be that the shoulder is getting bumped back a notch by the neck not expanding and sealing off the gas.

 

[BrokenWheel]

No- the die and the brass was clean.

 

[MEHavey]

It probably never blew the shoulder forward at all and the case simply saw a short pressure pulse.
I'm not surpised that the brass "low-pressure-bounced"* a hair to return to the same dimensions/slightly less.



*
Low-Pressure-Bounced: A new technical term just invented here

 

[gamestalker]

I think your on target MEHavey. Sub Sonic loads can deffinitly effect how, when, and where the brass expands. To achieve the most accurate chamber dimensions from the brass, I would recomend using a load that is running good average working pressures.

 

[243winxb]

Light loads or loads with slow powder will expand the body of the brass right behind the shoulder. This pulls the shoulder & neck backwards. At the same time, the neck diameter may not get larger. Try inserting a bullet into a fired case.

 

[Walkalong]

Yep.

 

[BrokenWheel]

Very interesting I didn't know that.

 

[Slamfire]

I set my headspace measurement to 1.462 on my 223 brass. On my AR it expands to 1.466". The other other day I was shooting subsonic (reduced loads) in this brass (trail boss). Measuring the headspace on these loads showed that the brass headspace had decreased to 1.461.

I double checked and re-zeroed my caliper, even made a small dent on the brass to see if the powder expanded the lower part of the cartridge (it did). despite all this the headspace still seemed to of decreased.

I'm not sure whats going on...

Gas guns are designed to unlock when there is still a residual pressure in the barrel. Chin calls this the residual blowback effect and I believe it is there to add additional time for useful gas pressure.

See the inset, that is the period during which unlock occurs in the 308 .



Of course the gas pressure cannot be so high that the case sticks too tightly to the chamber or the rim will be pulled off or the case will be ripped in half.

Roller bolts open so early in the pressure curve that the only way they can function without ripping the case in half is with gas lubrication. The upper 2/3 rds of the chamber is fluted so barrel pressure can float the upper 2/3rds of the case. The bottom 1/3 serves as the case seal.





But given that there is some gas pressure at unlock and extraction in your mechanism, and you don't have chamber flutes, it makes sense that the thinnest part of the case is going to be sticking to the front of the chamber, and that the case is going to get stretched during extraction.

And I believe that is what you are measuring with the standard pressure cases.

I started leaving the lube on my M1a cases, or rubbing loaded cases with Johnson paste wax to reduce case stretch. I don't have case head separations with appropriate powders and charges. The front end of my cases don't stick to the chamber during extraction and my cases do not get stretched.

I have run lubed cases in the AR but I don’t recall checking headspace of the fired case, though I know I have. I suspect they did not grow either.

Your light loads, I suspect unlock occurred when the residual breech pressure was too low to stretch the cases.

 

[eldon519]

In an autoloader, the bolt slamming on an overly-long case can bump the shoulder back. I have seen this in .308s. I have no reloading experience with ARs, but I do reload for an FAL and M1A. I think the AR-15 is a bit gentler in extraction, but the process generally does stretch the case, ie the chamber headspace is likely less than the 1.466" measurement they are popping out at. It might actually be less than the 1.462" you are setting them at. In an FAL and especially M1A, that can cause slam fires, not sure how common it is in ARs. I don't know how you could test it on an AR, but it's very easy on an FAL or M1A as you can turn off the gas sytem and fire them from the closed, non-moving bolt and get a pretty good idea of the chamber measurements (though the rear lock-up on an FAL does still allow some stretch).

Just a notion, could be something else as well.

 

[Kernel]

The phenomena of decreasing headspace is well known in brass that is used with reduced-pressure loads. At the moment I don't recall the exact physics (it's due, in part, to lack of proper obfuscation), but it is strongly recommend that you mark and segregate brass that's used in reduced-pressure loads and not use it again for full power loads. Once it's shrunk, it takes some effort to get it back to spec, without over-working the brass and/or putting unnecessary stress on the firearm. Not worth the hassle with common .223 brass IMO.

 

[243winxb]

the bolt slamming on an overly-long case can bump the shoulder back.

I have seen and measured this using an AR 15 type action. Its pushed back about .001"

I think the AR-15 is a bit gentler in extraction, but the process generally does stretch the case, ie the chamber headspace is likely less than the 1.466" measurement they are popping out at.

The chamber headspace would be more/longer (full Power loads) because of brass springback. Stretching of the brass happens because the shoulder has been pushed back to far on FL sizing. On firing, you may get a case separation in the head or body if the shoulder has been pushed back more than .009" when FL sizing.

 

[eldon519]

The chamber headspace would be more/longer (full Power loads) because of brass springback. Stretching of the brass happens because the shoulder has been pushed back to far on FL sizing. On firing, you may get a case separation in the head or body if the shoulder has been pushed back more than .009" when FL sizing.

I can only speak from my experience, not the theory. In an experiment I performed with my M1A loaded, I fired 20 rounds, 10 with gas on, 10 with gas off. These rounds were intended to roughly duplicate NATO spec with a charge of BL-C(2) roughly midway between start and max. The average of the cases fired without gas was 1.6321" measured with the Hornady tool. The ticket that came with my rifle indicates the headspace of this particular specimen is 1.632". The average for the cases fired with the gas cycling the action was 1.6342". My FAL also exhibits roughly 0.002" of case stretch when cycling with the gas system. It did not come with any headspace measurement from the manufacturer, and I have not attempted to measure it. I anticipate that more stretching might be seen with a max load or slower powder.

This is what I was referring to when I mentioned stretching. In autoloaders, at the moment the action starts to cycle, pressure is still present in the barrel, so the case is still gripping the chamber walls at the time the extractor starts pulling on the case head, hence the stretching. I agree though, excessive full length sizing will cause the case to stretch when fired as the pressure pushes the shoulder forward.

I personally expect that I have experienced more set-back during loading during my early development of my loads for the FAL. I know I have fed it handloads measuring up to 1.633" with the Hornady tool, and it has loaded them without problem. If the results from my previously mentioned experiment are to be believed, I'd expect it is setting the shoulder back more than 0.001", but the action of a 7.62x51 is cycling with more force and inertia.

 

[243winxb]

eldon519, your test would be more accurate as you can turn the gas off. Good info, thank you.

 

[Clark]

The .223 Remington is SAAMI registered at:

Cartridge headspace from 1.4596" to 1.4666"
Chamber headspace from 1.4636" to 1.4736"

I measured the head space [beyond the minimum] of my (5) 223 rifles:

a) Ruger falling block .003"
b) Colt AR .005"
c) Bushmaster AR .0065"
d) Wilson AR .008"
e) Mauser Bolt .008"

I got involved with this over case trim.
A big part of the calculation was the firing pin pushes the case forward. Hard.