New Brass 50fps slower than fired brass?

[Stainless87]

So I have been developing a big game hunting load for my 6.5 creedmoor savage lightweight. All of my work has been with brass that has been fired at least 5x times. I found my load and then loaded up some new brass. I always expect a little bit of difference, and usually end up dropping .1 to .2 grains to compensate for the difference in velocity and poi in new brass vs once fired. Same dies and reloading procedures being used on both. Same environmental conditions and same magnetospeed chrono.

Load:
6.5mm Creedmoor
Hornady Brass
Fed LR Match
Varget - 39.1gr
123gr Hornady SST
2.780 COAL

For this load, I got 50 fps reduction in new brass vs my 5x+ fired brass. I will need to add about .5gr of powder to get back up to my load development velocity. Some things I noticed POI shifted by 1/2 moa to the left, same for elevation. 4 shot groups went from .40moa to .9moa. Neck tension was noticeably less with the new brass. These were shot on a clean bore. Most load development took place with 20-40 shots in the bore and was exposed to some HBN bullets down the tube. Are you guys seeing this much difference and what is an explanation that can cause the dramatic difference in velocity?

I've been working a mod.70 243 pushing 90 grain ballistic tip sitting on 38 gr of 4064 @3005 fps on once fired brass FL resize .5 group with new brass lost 50fps opened up to a 1.5 group now I'm not sure what to do next

 

[Twicepop]

No carbon (lubricant) in the neck causing a harder bullet pull reducing velocity.

 

[mikecr]

No carbon (lubricant) in the neck causing a harder bullet pull reducing velocity.

No, it doesn't work that way. Higher seating friction, in itself, means nothing to neck tension nor pressure/velocity.

Anyone with a seating force gauge can test this and see it for themselves. Take one of 5 cases with normal internal neck fouling, clean it to shiny brass, & alcohol wipe. Go ahead and do the same for one bullet. When seating that clean bullet in that clean neck, notice higher seating force required to reach your desired CBTO.
That higher seating force, due to friction in itself, would also present a higher 'pull force'.
But now, when you fire that round with the other four, across a chrono, you'll see no difference in MV.

The reason for this is that your bullets are not pushed through neck friction on firing.
Instead, necks expand to fully release bullets, which are swinging in the wind by first movement.
For this, it takes nearly nothing of expansion to occur. Far lower than any of us could measure.
For all the fretting about needed & 'safe' neck clearances? It's hogwash. If you can chamber a neck without interference, so that a neck can expand at all, it will fire just fine. I've tested it.

If a neck needs so little expansion to release bullets, then this might be a pretty small force to cause it, right? Well, since expansion/release occurs before the load would have to overcome a pull force, then neck expansion must require even less force. So yeah, it's a really small force relative to that available.
But even the smallest things are significant with internal ballistics.

Neck tension,, what holds bullets, is spring back force against an area of bullet bearing.
We adjust spring back force with management of brass hardness, and the area of bullet gripped through sizing length. You can test this as well, with a bushing die that allows adjustment of sizing length.
This is not easy to see, until extremes, with a seating force gauge. Again, validating that pull force is not a good indicator of what matters. But you can see MV change with this.

 

[Ccrider]

Hunting load? The animal that you hit will never know.
Tweak the load tune with your new brass and go hunting.

 

[Zero333]

I've been working a mod.70 243 pushing 90 grain ballistic tip sitting on 38 gr of 4064 @3005 fps on once fired brass FL resize .5 group with new brass lost 50fps opened up to a 1.5 group now I'm not sure what to do next

Is it the same brand of brass ?
Is it the same Lot of brass ?

Are you comparing an average of several groups on 2 different days ?

Anytime you change 1 little thing in the load, you can't be surprised when it doesn't shoot the same.

Outside temp alone can rob you 50 fps and take your half moa load out of tune.

 

[searcher]

Do you record air temperature on the days you do your test loads? The difference between testing a load at 35 degrees and 95 degrees will do that. Makes a pretty big difference, though a lot of folks don't give it too much thought. To have a 50 fps difference from one session to another, whether using new brass or not - is not uncommon - and it may have little to do with the brass. Each time one exposes their powder to open air by uncapping the powder jug or pouring in and out of a powder hopper - the powder absorbs or ejects moisture content. If you opened a new jug - that alone could easily change 30 FPS - as I record velocity every time I open a new jug - even from the same lot. That is pretty normal - even using the same set of brass. You will get some variation regardless of brass manufacturer - some a lot and some almost the same - but usually a bit different. A primer change will usually vary around 15 to 30 fps, and so on. This is why many competition shooters load their ammo at the range just before the match. Not to worry.