diff in ES between virgin brass and once fired brass

[dmax1800]

I'm doing load development for a 300 win mag. Before I did my ladder test with Noser 180 AB and IMR 7828, I shot 10 sighters/foulers with virgin Norma brass that had been FL resized with RCBS dies. ES was 19 and SD was 7. The ladder test with once fired Norma brass had much more variance in vel, not just do to increases in powder. I've always had problems with relatively large ES (40 to 90) in group testing with the same powder charges. This is the only time that I have shot virgin brass before a ladder test or group test.
Question: why would ES be lower on virgin brass than once fired brass???
Thanks for your help.

 

[Mikecr]

To begin, it isn't virgin brass if you had FL sized it. At that point it was what your die made of it.
Then you fired the brass, and sized the brass even more, departing even more from virgin.
With every cycle of heavy FL sizing you change it more & more.

It's a big price.

 

[SidecarFlip]

+1. I'd have neck sized it and see if it chambered satisfactorily or even not did anything, just loaded it.

 

[Bart B]

Question: why would ES be lower on virgin brass than once fired brass???

Lower amount and spread of case neck tension on the bullets in virgin brass.

 

[MontanaRifleman]

+1. I'd have neck sized it and see if it chambered satisfactorily or even not did anything, just loaded it.

Unless it's Lapua brass, some or a lot of the necks will have irregularities, so I would always size prior to loading (unless it's Lapua and the necks were good). And neck sizng is fine... if you are a neck sizer. Some of us though, are FL sizers, and when FL sizing unfired brass, it basically doesn't amount to more than neck sizing cause any new brass that I ever bought was under sized in almost all dimensions.

 

[MontanaRifleman]

To begin, it isn't virgin brass if you had FL sized it. At that point it was what your die made of it.
Then you fired the brass, and sized the brass even more, departing even more from virgin.
With every cycle of heavy FL sizing you change it more & more.

It's a big price.

Are you advocating annealing?

 

[MontanaRifleman]

Lower amount and spread of case neck tension on the bullets in virgin brass.

Wouldn't that depend to what degree the necks are expanded.... virgin or not?

 

[Bart B]

Regarding my remark about the spread of case neck tension on the bullets in new versus once fired brass.....

Wouldn't that depend to what degree the necks are expanded.... virgin or not?

Yes, I think so. Depending on what dimensions a sizing die makes a fired case neck compared to an unprepped new case necks have, there could be a difference. For consistancy's sake, I think new case necks should be resized.

I've not seen any significance between bottleneck cases headspacing on their shoulders with new brass with "prepped" necks and once (or many times) full length sized ones as far as accuracy goes. Never chronographed both with the same recipie to see what muzzle velocity difference might exist. Both shot to the same point of aim with the same sight settings withing 1/2 MOA at 600 to 1000 yards in about equal atmospheric conditions.

Others may well have observed differences.

 

[idaho elk hunter]

I always recommend resizing. Virgin and spent brass. This allows the neck tension to be uniform from shot to shot. This is one of the MAIN factors in accuracy!
Second,,, it makes for more uniform and consistent loadings between the 1st and 6th use of the case..

 

[Mikecr]

Are you advocating annealing?

No, only when forming a change in a cartridge. New shoulder angle, cal change, etc.

OP's issue was never focused to neck sizing only. He's FL sizing.
Apparently FL sizing enough that his cartridge is changing in character.
Every sizing cycle on a large case body changes it's capacity a little bit, and some cases moreso than others in a batch. The greater the area sizing, the greater the spread from any standard in capacity.
This, because of variances in thickness & springback of the brass in the body. You can't anneal this.

It's one of many reasons for all the efforts out there to reduce sizing to a minimum.
And there is no getting around it, other than to stop resisting it.
You should load develop with brass -that has first been taken to stable chamber dimensions(fully fireformed), and THEN minimally sized as it should be to carry forward.
Only then can you find best load, or lowest ES load, etc., and reproduce it.

It makes no sense to fret over result differences between NEW brass and FIRED brass, regardless of sizing. You changed the brass

 

[MagnumManiac]

I also noticed this many years ago in the same cartridge and brass.
I think it's most likely caused by the difference in case capacities between unfired brass and fireformed brass, especially in Norma cases.
If you have a RCBS Precision Mic, it is easy to see that the case shoulder moves forward by as much as .062" in this brass, moreso than brass like Winchester and Remington which are significantly thicker than Norma brass. I have measured this difference between fired/unfired capacity to be over 1.5gr of H-4831, this is significant enough to change ES and SD because it alters the packing scheme of the powder in the case!
I learnt of a loading technique many years ago that you 'swirl charge' the cases by dumping powder into the scale pan and swirl the powder through/down a funnel into the case, I can get 83gr's of RE25 into a 300WinMag case using this method, if dumped straight from the measure it overflows and spills everywhere. The slower you pour the powder, the more that will fit, within reason of course.
This method has shrunk my groups when using heavily compressed loads by allowing better packing of the powder and more room for the bullet, the reason is due to a better burn shot to shot as proven on my pressure trace.

Cheers.
gun)

 

[Mikecr]

Load density is VERY important.
If you built a load with a charge of low load density(say 95% fill), and you cycle rounds from a magazine, your ES will be higher than rounds single shot fed and consistently/slowly chambered.

This could be one of the reasons OP had lower ES from new/smaller cases. His load density would have been higher at that point.
But this is just one of many things that needs to be taken off the table before load developing.

 

[MontanaRifleman]

Load density is VERY important.
If you built a load with a charge of low load density(say 95% fill), and you cycle rounds from a magazine, your ES will be higher than rounds single shot fed and consistently/slowly chambered.

This could be one of the reasons OP had lower ES from new/smaller cases. His load density would have been higher at that point.
But this is just one of many things that needs to be taken off the table before load developing.

I don't think that's it. In loading and shooting the 300 RUM I have not seen any more than maybe 1 gr difference in case capacity between once fired and virgin brass. I have also had good luck with low density loads in that chambering, 92-93%. Also 1 full gr diff in powder is equivalent to maybe 30-40 fps of velocity, not 80.

This is a puzzling situation and I suspect that somewhere in the process there is something going on to cause significant pressure variations. Without being able to actually view the OP's entire process we are blind and just speculating in the dark.

 

[Mikecr]

It's probably true that we don't have enough info.
My thinking is if the ONLY change was fired -vs- unfired, it leaves little other than capacity & load density.

Also, about you load density, try this:
At the range, load a shell with the barrel pointed up, carefully lower to rest, and fire across a chrono. Note that your MV went up.
Now do the opposite, with barrel pointing down on loading.
Note that the MV goes down.
Do the same test with fast chambering -vs- slow, and watch your ES.

I think you'll find that it's a good idea to develop with the fastest powder(for capacity) -that fills a case well.
And whatever load you've hit on in low case fill can be matched in higher case fill, using QuickLoad.