All else being equal, will the same load produce same pressures if loaded in new vs once fired, neck sized brass? If not, which will be higher. I am looking at working up loads for my 264 win in new brass. I am wondering if my results will be valid with once fired brass, from my gun, later. What say you? I know the new brass has more room to expand to fit my chamber, but I don't know the implications of that regarding changes in pressure.

I think you're over thinking things. There is a diffence but I don't don't think it's enough to cause any dramatic changes in anything. While I wouldn't shoot a match or even go hunting with a mix of new and 1x loaded ammo, I have worked up a load with new brass and then reloaded it and gone hunting. the fact remains that there is a limited amount of space in the chamber. When that is filled, it is filled.

IME the decreased case capacity increases velocity, hence it probably increases pressure. I've heard other guy claim they've had the opposite experience, and the soft new brass absorbes some of the shock, reducing pressures. I have a feeling the reality is actually pretty complex and may vary according to brand of brass, and where you are on the pressure scale.

If component prices ever return to a reasonable level, it would be fun to run some controlled experiments and see what variables a guy can isolate.

I have never found there to be a noticeable difference in pressure with new brass vs once fired brass of the same brand. Accuracy can be different and usually in favor of new brass in my experience.

Once fired neck sized brass usually fires like loads FASTER for me than virgin.

Since the chamber volume is the same, if the powder charge, seating depth, etc... are all equal, pressures will be equal as well.

I don't know if I'm reading this right. I assume we are talking about the same brand of brass? The case is only a way to hold components together. It will expand to the chamber dimension. The dimension of the case, walls ect, take away from the dimension of the chamber. Both the once fired and new case will expand as far as the chamber will allow. The only difference will be the case wall thickness.

Never develop loads with new, unfired brass

Fire form it first either with bullets or Cream Of Wheat.

I would say that once you work up your loads with new brass and get one that shoots well, the second firing will be more accurate especially with just neck sizing. If you are concerned about more pressure, drop your charge by a half of grain and see what it does. Personally, I believe the pressure difference will be nill between new brass or once fired, at least not enough to worry about.

I am always glad to get all my new brass fire formed. My second firings always are more consistant in all repects, especially nowadays when most new brass is has little uniformaty.

There's an easy way to answer the question. Load some up in new brass, fire it over a chronograph, and then do the same with once-fired.

That won't prove anything, as results by a few fps from day to day for several reasons, including light conditions, temperature and chance.

Pressure will be the same with either now or used brass of the same lot. This has been proven umpteen zillion times in pressure labs.

Obviously. So if he did that, and got a variance of only a few fps, he'd have his answer. Plus, who said he has to do it over two or more days, he could do it on the same day, and even alternate shots to account for variation in environmental conditions and fouling/temperature of the chamber and barrel.

My only point was, why ask a question and get a bunch of answers based on what people think might happen, when you can easily find the answer yourself.

Exactly, physics hasn't changed much....lol

This is my xperience, also !
Kraky is correct, IMO.
Jim

I believe that`s due to their being closer to chamber size and not wasting pressure on expanding the case and instead pushing on the bullet with more it.

Fired brass gives slightly lower velocities for me, hence the need for sizing dies, the case is larger after it's fired so pressure with the same powder charge is less.

Gunner

So now we have conflicting evidence from personal chronographs in different conditions.

Looks like "tastes great, less filling" to me.

Let's hear some more evidence from various people who use various chronographs under various conditions. That ought to settle the question once and for all.

May be that i only slightly neck size?

Gunner

I fondly remember the late Bob Milek writing about his Remington XP100 chambered in .250 Savage and its favorite load of a 100 grain Nosler BT with H335. Without fail he always cited the velocity as exactly 2705 fps, as if that 5 fps meant something. When I chrono loads, I usually only view the first two digits as significant figures, whereas the latter two are often vagaries attributable to living in a random universe. For example, when I test loads in my Swift using 55-grain bullets, if I get really good accuracy and the chronograph shows 38XX fps, I call it good. It doesn't matter to me if the average of several shots is 3789 or 3814, I still read it as 3800 and go on my merry way; because I know tomorrow it might be 3825 or 3794 or whatever and I'll be just as happy and the groundhogs just as dead.

Some may insist on extreme spreads less than 25 fps and s.d.'s in single digits, but I've got more important things to worry about. I try to keep track of the number of times my brass has been fired and relegate them to the same lots, but I don't think the difference in speed or accuracy (or pressure) is measurable within my limitations.

Does this mean that for the average guy with the average chronograph, pressure does not equal velocity?

If you're referring to my post above, I wasn't talking about "various" anything. it was one person, with one chronograph, under one set of conditions.

I would guess that fired brass would have slightly greater dimensions in most directions. I've never made any measures on bottle necked cartridges, but with my 45-70/90 straight walled units, where I use compressed loads, I can get several more grains of powder into fired cartridges without belling the case. In those same instances, the cartridges also end up slightly shorter after the first firing.

Fired brass might be a bit bigger on the inside, but the chamber itself is the pressure vessel and that's what determines the ultimate volume. Brass can easily be bent with your fingers so I doubt any real energy is wasted blowing out a case on the first firing. I don't think it makes any difference whether it's once fired or not. At least I've never been able to tell the difference over the chronograph.

I've tested this in a 308 Winchester with an Oehler 43 unit. The difference of pressure between new and once fired was within the normal variation of pressure that is seen within either.

Thank you.

When I saw my diffs it was doing alot of work with a 300 wby. Maybe I was crazy...and maybe I wasn't but it seemed like about 30 fps diff between virgin and once fired. Apparently Jim Hundley has seen the same he shoots a ton of 300 win mag. Is it possible it might involve the energy blowing the shoulder forward...there can be significant movement on belted mags on the first firing. Also seems to be a posting every so often about mysterious pressure spikes on 3-4 times fired mag brass and the logical answer as to why isn't found.
There wouldn't be alot of pressure diff for 30 fps in either 300s. Can't get to the range now but will have to re-experiment with this sometime. Maybe this is somewhat gun specific...kinda like. 005 run out destroys accuracy in one gun but doesn't bother the next.

+1

As Cleftwind mentioned, PHYSICS don't change!




(unless one's a member of the global warming crowd, who like the ozone layer

[alledgedly .... NOT!]
"changes"
day to day depending on where the wind is literally blowing!

I have experienced approximately 75 fps. increase in velocity when comparing virgin brass to once fired. Don't know why ? My theory is that the extra pressure is directed to the blowing out of the new brass to form to the chamber instead of all the pressure behind the bullet. This is my experience.
The rifle chamber is minimum SAAMI spected with a short leade. Don't know if that might have some bearing or not ?
This occurs with Nosler, Winchester, or Lapua brass. Identical loads.
Would like to get some opinions of why this occurs. I test over an Oheler 35.
Thanks,
Jim

Kraky, I don�t doubt what you say, and I don�t doubt what logger or Jim said above either. It makes sense to me that some may observe a difference of a few tens of fps, and others might not. A velocity difference of 30 fps is only 1 %. And it makes sense that the most likely place you�d be able to see a few tens of fps difference would be with fast cartridges (where a few fps is a smaller percentage of the total velocity) having a big difference between the case and chamber size like you described�more room for expansion.

I think it�s possible to see a variation of a few tens of fps without seeing an increase in peak pressure because velocity is not just peak pressure, it�s also duration and the area under the pressure curve.

And I understand the concept of the chamber being the pressure vessel (not the case) but my question to the guys who bring this up is, how much time does it take for the case to fully expand, and how does this relate to the time duration under the pressure curve? If you don�t know this, then I�m not sure you can conclude that case expansion does not affect velocity/pressure. Case expansion is not instantaneous, just like the rise and fall of the pressure curve is not instantaneous, so if the case is still expanding while the pressure curve is being generated, that could have an effect on the pressure curve, however small.

So for those who say "physics don't change" explain the physics of that.

If you noticed the way I avoided the pressure question in my original post I only said "faster" cause I had no way of knowing if pressure was necessarily affected. In all honesty I was seeing 30-50fps difference but stated the lower range as to avoid going down as a campfire crazy.
I was glad when Jim stepped in with the 75 fps figure.
I know Jim is meticulous and shoots his 300 alot so I know I'm not ready for the looney farm yet.
I'm dealing with a bad family illness and won't get to the range for a while but do quite a bit of loading for a friend with a custom 7 mag and custom 7 stw....he writes down the velocity of every shot...I will have to watch trends.p