POI change to once fired brass?

[MuleyFever]

Group on the left is new brass and the one on the right is once fired. The fired brass is neck sized only. Is this POI shift normal?

 

[Wrench]

One variable is neck tension. Some bullets like the lrab are very finicky about neck tension.

With that said, being that the groups are both quite good, I'd be inclined to look at my optics and if possible run it across the chronograph or some 700+ shooting to verify the velocity hasn't changed.

 

[JakeSCH]

I would say that is expected results. Once fired brass changes the internal volume, especially since you only neck size it.

It would be good to measure headspace of before / after fired brass to check. In my 6.5 PRC my once fired brass increased velocity by 50 fps or so with the same amount of powder.

 

[Dcrafton]

Group on the left is new brass and the one on the right is once fired. The fired brass is neck sized only. Is this POI shift normal?

View attachment 218427

First, is the once fired brass from the firearm your are using it for?
Second, new brass can have a different POI until it has been fire formed to the rifle chamber.
I have 2 rifles of the same caliber and I keep the brass separate from each other as they will have a different POI in each other rifle.


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[MuleyFever]

I’m using a Nightforce, maybe 50 rounds on it now. Unfortunately my chronograph seems to have died. Brass only sees one gun. Both brass neck sized with a bushing die then mandrel.

 

[Dcrafton]

I’m using a Nightforce, maybe 50 rounds on it now. Unfortunately my chronograph seems to have died. Brass only sees one gun. Both brass neck sized with a bushing die then mandrel.

Fire form brass before comparing it to other brass. New brass and once fired brass are not equal.


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[MuleyFever]

That was my first time using the fired brass. I will shoot it again of course but was just curious if this is expected.

 

[Dcrafton]

That was my first time using the fired brass. I will shoot it again of course but was just curious if this is expected.

Looking at your groups from each brass group, I would believe your setup is working fine. Good groups but different POI. I would start with equal fire formed brass and go from there.


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[CApighunter]

This is why I don’t neck size only. I full length size virgin brass and the same with every reload. Never had a discernible shift in POI.

 

[MuleyFever]

This is why I don’t neck size only. I full length size virgin brass and the same with every reload. Never had a discernible shift in POI.

So you run new brass through the sizing die before the first firing?

 

[Dcrafton]

So you run new brass through the sizing die before the first firing?

Yes, all standard dies (not crusty) are to SAAMI specs. I run all my brass through my dies before loading.


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[Ram94]

The way I'm making sense of this in my head, is with virgin brass some of the energy is transferred into expanding the case walls to the inside of your chamber. With the fired brass, that doesn't happen to the same degree, so you are getting more energy transferred to the projectile increasing it's velocity and thus its POI is higher.

 

[JakeSCH]

The way I'm making sense of this in my head, is with virgin brass some of the energy is transferred into expanding the case walls to the inside of your chamber. With the fired brass, that doesn't happen to the same degree, so you are getting more energy transferred to the projectile increasing it's velocity and thus its POI is higher.

I think it has more to do with the change in internal case volume than it does working the brass.

 

[Ram94]

I think it has more to do with the change in internal case volume than it does working the brass.

I'd say that does change things to a degree, but wouldn't that take the POI in the opposite direction if it was the major factor at play? The same charge weight in a smaller volume will generate more pressure which makes the opposite true as well. The same charge weight in a larger case will generate less pressure.

If we say that the explosion occupies the same volume in both scenarios (ie. the chamber dimensions) except that with the virgin brass, there is a dampening effect due to the losses to brass expansion and with the fired brass, the explosion still occupies the volume inside the chamber with less of a dampening effect then this could explain the increased velocity, no?

 

[JakeSCH]

I'd say that does change things to a degree, but wouldn't that take the POI in the opposite direction if it was the major factor at play? The same charge weight in a smaller volume will generate more pressure which makes the opposite true as well. The same charge weight in a larger case will generate less pressure.

If we say that the explosion occupies the same volume in both scenarios (ie. the chamber dimensions) except that with the virgin brass, there is a dampening effect due to the losses to brass expansion and with the fired brass, the explosion still occupies the volume inside the chamber with less of a dampening effect then this could explain the increased velocity, no?

Thinking about it more, I don't think we are saying completely different things so I apologize for the "correction". We both agree that the process of changing the shape of the brass causes a change in velocity. I was visualizing the change of internal volume during the brass expansion to cause a drop in pressure effectively changing the pressure curve phasing where you referred to it has a loss of energy...

Either way, it is a fun topic to think about and would be cool if someone had a pressure trace II to measure the shape of the pressure curves no matter what the source of the change of shape.

 

[Ram94]

Thinking about it more, I don't think we are saying completely different things so I apologize for the "correction". We both agree that the process of changing the shape of the brass causes a change in velocity. I was visualizing the change of internal volume during the brass expansion to cause a drop in pressure effectively changing the pressure curve phasing where you referred to it has a loss of energy...

Either way, it is a fun topic to think about and would be cool if someone had a pressure trace II to measure the shape of the pressure curves no matter what the source of the change of shape.

Agreed! There is a ton to learn and it's conversations like this one that help that along. It is mind blowing how much science takes place in the split second after the hammer drops haha

 

[CApighunter]

So you run new brass through the sizing die before the first firing?

Correct. New brass still gets sized, trimmed if necessary. The point being the case starts with the exact same dimensions every time.

 

[orelkaddict]

Yes this is normal. I fire form my brass before I ever "sight" it in. I have 5 different 308's and the brass never gets mixed up in different guns. Now say you only shoot factory ammo so its always new brass you fire so in theory it should shoot the same all the time

 

[Muley Buck]

This is why I don’t neck size only. I full length size virgin brass and the same with every reload. Never had a discernible shift in POI.

I've neck sized virgin brass all the way up to 6 times fired brass and didn't experience any POI shift.

 

[Lawnboi]

How much did your brass grow on the first sizing and how much did you size it?

I don’t think it’s uncommon to have to adjust charge weight a bit to achieve the same on new and once fired.

Same neck tension? Did you clean the brass thoroughly as if it is new? Any carbon left in the neck?

First thing I would do is compare velocity.