More details, the cases are all weight sorted and come from the same batch.
Another thing that I have noticed is that the "new" unfired cases have rounded shoulders while the once fired "fire formed" cases have nice sharp angular shoulded. The fired cases also end up being a tad shorter than the new unfired brass.
I assume that the fire formed brass would then have a slightly larger internal volume than new brass.
Could this fire forming process also be using up energy that would otherwise go into producing velocity? Either way I am definately getting a lower POI with new brass at 1000 yards.
Ok, in some circumstances you are right. Heavier cases dont always meen less volume. You are 100% correct. So lets assume that when I was refering to heavier cases I was refering to the ones that are thicker.
That said, I have experience the opposite as you. Using top loads in my 338 Edge using one case brand and lot and using the same powder charge with 'thicker' walled cases from a different lot#, I have has sticky bolts and cratered primers along with higher velocities. Going back to the original cases manifested average velocities and no sticky bolt. Granted I dont have a pressure trace but it seems clear to me that there can be differences. Espescially since it happens to other shooters and me with other guns in similar circumstances. Of course like alot of other things, it is relative. If you are not using max loads and the difference can be much less. The closer you get to maximum pressures, the more sensitive things can be problems become compounded due to the 'little things'.
Call it what you want but there are circumstances where thicker cases can and do cause higher pressures.
Had the same experiences with my edge and .308. The closer to max pressure the more critical. Good grief, don't take a near max load in .308 Remington brass and load it in Lake City brass. That small amount of case volume makes a big difference. This doesn't matter with my 7mag, but I also load it rather mildly. It doesn't care if I load up Winchester or Remington, brass or nickle. I stopped turning the necks on it as well, it doesn't care what I do to the brass...or what primer I use! As long as I feed it a 162gr SST over 68grs of R25.
I was fireforming some brass in my 338kahn a few days ago and noticed a similar situation. There is quite a bit of difference between a factory 338/378 and the formed Kahn. I was shooting both forming rounds and brass that had been previously been formed, both using using the exact load. Every round fired through an Oehler chrono. Formed and unformed brass both had necks turned.
The forming loads were 35 to 40 fps less than the pre formed brass every time. I found this rather curious since the unformed brass has less case capacity. The obvious answer to me is that the forming process uses a fair amount of the power contained in the powder charge to form the case, resulting in lower velocities. Of course there is much more brass to move around in forming a 338Kahn from a 338/378 than in your example so I would expect your velocity reduction to be much less.
this is interesting. i'm trying to digest whether a case would "rob" some of the pressure/energy to fireform. my guess is a case is smashed out against the chamber while the bullet is engraving into the rifling when peak pressure is occuring and long before the bullet exits the barrel. it doesn't seem logical but does make sense to explain the difference in velocity.
I agree...... it is "interesting". I had never thought about it before, and really did not give it a lot of thought when it happened. When my example happened to me I was shooting "once fired" formed brass and unformed brass from the same lot and prepped in exactly the same fashion.
I normally would not be shooting formed brass and fireforming brass in the same shooting session so I never noticed this before. I was logging each shot velocity and noticed that while my average speed with the once fired formed brass was 2875, the average speed of unformed brass was 2835. This was over about 20 rounds of formed brass loads and 10 rounds of fireform loads and very consistent.
I just kind of shrugged my shoulders and thought "interesting". Then this thread comes along with a similar situation..... interesting......