Does Unfired vs fire formed brass make a difference?

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I recently asked the question above on a thread a while ago, I got some mixed answers but the consensus seemed to be that there would be a difference between fire formed brass and once fired brass in terms of velocity, all else equal.

Obviously Im not talking about blowing out some Ackley Improved brass or necking up/down some cartridge, or even some of these chambers where they use a No-Go gauge as the "go" and a field gauge as a "no-go" to kind of game the system in competition. Im simply talking about firing a piece of virgin brass made for the chamber you are shooting, and then either neck sizing, or setting the shoulder back a couple thousandths.

Also, Im not taking about accuracy or extreme spread reduction...just average velocity.

I experimented with .223, .308, and .300 win mag (as you can tell from some of my previous posts, im on a .300 wm kick lately). In the .223 I used RP and Winchester Brass. In the .308 I used Federal Premium and RP brass, and in the .300wm I used Norma brass only. So obviously this isn't a comprehensive check as I didn't use every manufacturers cartridge, and I only used 1 powder type - Varget in .223, IMR 4064 in .308, and H1000 in .300 wm.

I tested each cartridge in the same chamber, and on the same day, and alternated shots with once fired vs fire formed brass. I did the .300 wm on a separate day than the .308 and .223. All done in the morning when it was nice and cool. I did 50 rounds each for the .223, and .308 and 20 rounds for the .300 wm. All the once fired brass was annealed.

With respect to average velocity, the findings were unremarkable. In the .223, the average velocity went up 5 fps from new brass to once fired. In .308, the average in new brass was 7 fps higher than once fired, and in the .300 wm it was only 3 fps. I don't need a calculator to tell you that those deviations are not going to be statistically different from 0.

Now, the groups in once fired brass were tighter, and the ES were down...especially in the .308 in Federal brass going from 36 fps to 11 fps.

So what does this add to the body of knowledge? I guess I'm not sure. A reduction of extreme spread in fire formed brass is not any revelation, that is in fact one reason why we fire form brass. But I think that this may show that changes in you velocity from virgin to once fired brass could depend more on the day you choose to shoot and the environmental conditions than anything else....unless of course you have a sloppy chamber, which I do not in any of my guns. Or, the increased velocity people see in the 2nd firing and beyond could be because they do not anneal and the changes in bullet hold force are modestly increasing the overall average velocity of their pet load.

I think though that you can go ahead and do a load workup in virgin brass, and won't have to do a second workup in it once it has been fire formed, just tweak it a little....which I suppose makes the cream of wheat method a waste of time unless again, you are making AI brass or something.
 
either neck sizing, or setting the shoulder back a couple thousandths.
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In your test, what one was used?

Neck meaning , only the neck was sized?

Setting shoulder back, meaning in a FL die ?

A military test showed a 4% increase in volume from new to fired brass, 5.56mm.

http://www.dtic.mil/dtic/tr/fulltext/u2/a456635.pdf Page 29
Assuming the brass case does not relax after firing, the case volume increases
by about 4%.
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Very interesting post. Ty.
 
This is the sentence from the article you linked to.

The previously fired case is of particular interest because it represents a true operating chamber volume. Assuming the brass case does not relax after firing, the case volume increases by about 4%.
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The first thing to note about the article is that they were measuring un-sized fired cases in a military chamber. NATO chambers from what I understand are more generous, especially if the "gun" in the article referenced was a belt fed machine gun. I found no reference to the gun used...Even if it is a Mk 18 capible of burst fire which is what I carried in the Navy, the chamber is larger to ensure proper feeding and if you have ever tried to resize military brass fired from a belt fed or a SMG, you know you need a small base or body die.

So I'm not sure you can glean anything meaningful about case volume from that report, as that was not its focus.


As for your question on sizing, those are good questions and a terrible oversight on my part. When I resize I use a Redding competition shell holder set and move the shoulder back 2-4 thousandths with a standard FL die. I have also neck sized in the past. However I no longer neck size. But my old chronograph data from a .270 win I used to necksize in suggests no remarkable difference.
 
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New brass is nearly always undersized so it will fit in the smallest SAAMI chamber. It will depend on the diameter and length of your chamber on how much the volume of your case will grow but it will nearly always grow to some extent. Because of the volume difference it will take a different charge for the best accuracy than a new unfired case.

As far as a resized case versus a new case it will again depend on the size of the die and how closely it matches your chamber. A die that oversizes the brass can make it enough smaller than the chamber that it promotes case failure. A die that is made with the same reamer as your chamber will only minimally resize the brass keeping the volume much closer to the chamber.
 
When loading the same amount of powder, new or FL sized cases will give higher velocities compared to Neck sized cases mainly because the burning chamber is bigger in the latter case. You therefor have to adjust the powder charge to get the same velocity.
 
Using a No-Go gauge as the "go" is a good way to do ugly things to your property. Headspace gauges check tolerance and nothing else.
Anyway, the only difference the brass makes is what you need to do to it before loading. BNIB brass needs to be checked for length, trimmed as required(usually not at all), the mouth chamfered and FL resized.
Once fired out of your rifle, needs to be checked for length, trimmed as required only, cleaning and either FL or neck sizing.
Once fired out of another rifle needs to be checked for length, trimmed as required only, cleaning and FL resizing.
"...new or FL sized cases..." Have nothing whatever to do with velocity. Neither do neck sized only cases.
"...Assuming the brass case does not relax after firing..." If it doesn't it'll be stuck in the chamber. Brass' elastic properties is why it's used.
 
Using a No-Go gauge as the "go" is a good way to do ugly things to your property. Headspace gauges check tolerance and nothing else.
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This is what some F-CLASS TR guys do to thier .308's to kinda sorta make a .308 improved..So to speak. It is gaming the system a little.

Anyway
This next quote
Once fired out of another rifle needs to be checked for length, trimmed as required only, cleaning and FL resizing.
"...new or FL sized cases..." Have nothing whatever to do with velocity. Neither do neck sized only cases.
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Is basically what I found out. Again, if you have a loose chamber, then maybe it makes a measurable difference. But I think ambient temperature, humidity, etc will mean more than the small changes in volume. Also, your accuracy window shouldn't be that short where 5-10 fps will open your groups up. If it does, then you need to go do a ladder test to find a bigger plateau.
 
I recently asked the question above on a thread a while ago, I got some mixed answers but the consensus seemed to be that there would be a difference between fire formed brass and once fired brass in terms of velocity, all else equal.
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I am a case former, I form first then fire. And then there are fire formers, they chamber a round, pull the trigger 'and' wh-la! They become fire formers. When I chamber a round, pull the trigger the case becomes a once fired case.

So; what factor changes, what is the difference between a minimum length/full length sized case and a once fired case when fired in the came chamber?

F. Guffey
 
Unless case volume varies due to a variation in wall thickness, the initial difference between a fired case and a new case will disappear in a microsecond. It conforms to the chamber. The brass is a seal, not a pressure vessel. That's why you find a negligible difference, Mississippi.
 
Mr. Guffey is correct. Brass is yielding to pressure by the time the bullet is let go of. It happens around 12,000 psi, typically. For lower pressure cartridges, if the peak is below about 30,000 psi—the brass has generally slipped back in the chamber and not expanded hard enough to stick. In these rounds the resized brass volume has more influence on start pressure and from that, final velocity. For anything over about 30,000 psi peak pressure, you are looking at the as-fired size being the better predictor, so the chamber.

About the only difference I would expect to see would be that a larger case not fully resized will have a little bit more air space at the start of ignition. It could affect the speed of the pressure curve growth a little bit, and that could possibly account for the small velocity change you see. Unfortunately, other things could, too. Your case cleaning method, for example. There's been a lot written now about how stainless pin and ultrasonically cleaned cases tend to stick harder to a bullet if an inside neck lube is not used. That could increase velocity a little. I would suggest taking some once-fired brass and full length resizing some and doing what you have been to the others. Same lot, same load history, same cleaning treatment. See if your difference is still there?

What sample size are you using?
 
About the only difference I would expect to see would be that a larger case not fully resized will have a little bit more air space at the start of ignition. It could affect the speed of the pressure curve growth a little bit, and that could possibly account for the small velocity change you see. Unfortunately, other things could, too. Your case cleaning method, for example. There's been a lot written now about how stainless pin and ultrasonically cleaned cases tend to stick harder to a bullet if an inside neck lube is not used. That could increase velocity a little. I would suggest taking some once-fired brass and full length resizing some and doing what you have been to the others. Same lot, same load history, same cleaning treatment. See if your difference is still there?

What sample size are you using?
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With respect to sample size, do you mean number of shots each? If so, it was 50 rounds for each group, except for the .300 wm which was 25. Not a big sample indeed, but if there was something dramatic going to happen, I would have seen it. In statistics a sample size of 30 is generally accepted as the minimum to begin reporting descriptive statistics (or 10% of a population in a random sample...whichever number is larger).

The cleaning method you bring up is an excellent point. My velocity extreme spread in un-lubed, stainless steel pin cleaned, cases, is relatively large in any caliber. But a light dusting of graphite, or talcum powder around the shank-boat tail junction does the trick. In fact, I forgot to lube the first 5 .300 wm bullets I seated and some of them reached velocities as high as cases loaded with a full 1.5 gr more powder!!!! It isn't a problem provided you can ensure it is consistent, which it isn't. Hence my bullet lube step.
(I didn't include unlubed bullet data btw.)

Along those lines, I may begin tumbling my brass in dry corncob or walnut media to polish the brass before bullet seating. Maybe this will remove the need for a bullet lube step.
 
Unless case volume varies due to a variation in wall thickness, the initial difference between a fired case and a new case will disappear in a microsecond. It conforms to the chamber
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So; what factor changes, what is the difference between a minimum length/full length sized case and a once fired case when fired in the came chamber?
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Unless case volume varies due to a variation in wall thickness
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The volume increases when fired, believe it or not it takes time for the powder to ignite and pressure to build. The case must expand and fill the chamber. If the case is neck sized forget the time factor because it is not necessary for the case to fill the chamber.

F. Guffey
 
50rd samples are a very good sample size and IMO you have put to rest the velocity issue with respect to fire formed verses not . What was the ES/SD of those groups ?

The one thing I'd be thinking about it virgin brass vs FL sized ( minimum shoulder bump ) vs fire formed ( neck sized only ) . With out ever running the test my self I would think the Minimum shoulder bump and neck sized only would test closer to the same then the FL virgin brass . I have some virgin 308 lapua brass that measures from head to datum point .003 to .004 shorter then I FL size my cases which would make them .005 to .006 shorter then fire formed cases from the same rifle . I'll actually be testing if bumping the shoulders back .002 will produce the same accuracy and velocity results as my test did during initial load development with virgin brass .

I've been very concerned that working up a load using virgin brass will give a false result because those cases will never be dimensionally sized the same again .

So I'll be testing that on the 10th of next month because I now have enough once fired and virgin Lapua brass to run a comparison test .
 
Mississippi said:
Along those lines, I may begin tumbling my brass in dry corncob or walnut media to polish the brass before bullet seating. Maybe this will remove the need for a bullet lube step.
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You might just take a couple of turns inside the neck with a bore brush. This fellow was doing that. I understand the "Froggy's Lube" referred to in the article was just graphite powder in alcohol.

Your sample size sounds great. I'm just used to so many folks only firing 3 rounds of something and thinking they've learned all they can from that. I forgot you work with stats (I am remembering that correctly now, right?).
 
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Your sample size sounds great. I'm just used to so many folks only firing 3 rounds of something and thinking they've learned all they can from that. I forgot you work with stats (I am remembering that correctly now, right?).
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yes. I have a PhD in economics and a PhD cert in statistics.... Which means I didn't write a 2nd dissertation. But I am an economist for a living doing stochastic and dynamic programming as well as some basic linear regression stuff.

Anyway I digress. Meta God.: I am at my in-laws house away from my notes but I can tell you that the extreme spreads were low in the once fired brass and standard deviation in the single digits. I don't recall the standard deviation of the Virgin brass off the top of my head but it wasn't much higher. However, the extreme spreads WERE significantly higher in Virgin brass with the exception of the Norma brass in my .300 wm. Honestly the once fired and new brass loads were so close you could chalk the difference up to not letting the barrel cool ALL THE WAY down or me not having my chroney absolutely perfectly in line.

Again though I want to stress that a chamber at max SAAMI headspace or one cut with a fat tool might in fact see a remarkable difference in average velocity. But I'm just not finding it.

But what certainly does happen is that groups tend to tighten up and extreme velocity speeds move closer together. I wonder if folks who are adamant that velocity increased with once fired and re-sized brass or velocity definitely dropped either didn't have a big enough sample when they first developed the load, or conditions changed a little bit.
I mean, all it would take is carefully trying to aim on the 5th shot of a 5 shot group fired in relatively short order with a temp sensitive powder and boom! The average goes up 15 fps. And if that is the only group you tested to see if your load you developed the first time through has changed then....

When I am testing loads for any reason, I always shoot through a chronograph. It is amazing how lighting, distance from the muzzle, and not having the dang thing perfectly straight will change things....So much so that I built a big canopy for my chroney and I use LED's for light so that if I'm off, hopefully I'm consistently off the same magnitude and always above or below the true velocity. Pretty much all chronographs I know of are basically timers that count how long it takes a projectile from the first sensor to the second. And if it's setup just a little crooked, or you are shooting at a target that is above or below the muzzle down range, which would increase the distance the bullet has to travel to get from the front sensor to the rear, then you could document a velocity increased or decrease when in fact none existed.

Which is why is shoot a lot at 200 yards at my own range I built at home. otherwise I look like I'm headed out in a massive scientific expedition with all the test equipment, levels, lasers, note books etc.
 
Years ago I was fireforming cases, .222 out to .222 Gleek (sharper neck angle for more capacity). While fireforming the bullets were seated out very tight to lands to hold the case tight against the bolt face and a light load was used. I shot my best group ever while fireforming, .106 in my benchrest heavy varmint gun.
 
Not to try and beat a dead horse, but I have one more observation to add.

Next weekend I have a 600 yard exhibition match I shooting in. Since the points won't be reported, I'm using the opportunity to practice AND fireform some new brass. It is from the same lot as some twice fired stuff I have. I loaded it the exact same as the load I finalized in fireformed brass.

Today I went out an fouled the bore and zeroed the rifle at 200 yards. Not counting the two first shots which were way slow due to a clean barrel, the velocity was 5 fps slower on avg than once fired (note: only 8 shots so small sample) But! It was 20 degrees colder and cloudy.

What is absolutely true though is that the extreme spreads are bigger in Virgin brass...26 fps today across 8 shots vs single digits in the other Hundred once fired batch. And the one that was too slow and expanded the spread was the 9 the shot warm bore.

Anyway. It just supported my original claim so thought I would post it.
 
I recently asked the question above on a thread a while ago, I got some mixed answers but the consensus seemed to be that there would be a difference between fire formed brass and once fired brass in terms of velocity, all else equal.
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I am the fan of forming first and then firing. When I eject a case after firing it is a fired case, to everyone else it is a fire formed case. Again; I am the fan of off setting the length of the chamber with the length of the case from the shoulder of the case to the case head.

F. Guffey
 
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