Home Forums Reloading Big Game Rifles New brass vs once fired...all else being equal.....do they yield same pressure?

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