Change in velocity from new brass to once fired "fireformed"?

[T3_tinker]

Ok, so pretty much as per the title. If I were to get a muzzle velocity for first loads with new brass, would there be a big difference in the velocity once the brass has been fl sized and reloaded? Also same for bush sizing?

[Brillo]

I have never observed a measured difference in MV between new and fire formed cases.

The pressure limiting factor is the chamber, not the brass.

 

[VarmLR]

I have had differences between new and fireformed, with higher velocities from fire formed in each case (up to 50fps for the same load).  I think a lot depends on your chamber and the energy expended in fire forming the brass (but may stand corrected here as I'm unsure of the exact reasoning) rather than propelling the projectile.  Rule of thumb purely from my own reloading notes shows a clear difference, never so more marked than with my 6.5 CM loads (initial MV with new brass was 2680 to 2700fps and 2750fps give or take for the fire formed for the same load).  I guess it also depends on the brass used to start with.  To caveat my experience, that was using Lapua brass.

[DaveT]

If as you say the brass is FLS after firing then its not really fire-formed but put back to somewhere near original dimensions so pressure would be about the same.

If neck sized only  and fired  in a generous chamber then you may see some pressure reduction.

[Mattnall]

As you trim the brass the wall thickness will change and therefore the internal volume will also change, even with FL sizing.

This in turn will change the burn characteristics.

But by how much and would it make a difference or even be measurable I wouldn't like to speculate.

[Brillo]

I can only repeat my first reply in that I have never observed a measured difference. 

I shoot F Class, both Open and F TR, so the only calibres I’ve taken velocity readings of new and fire formed are .308 and .284, and with Lapua cases.

The op is reall asking a rhetorical question because the only way he/she is going to get a satisfactory and conclusive answer is to test both.

[T3_tinker]

Thanks guys. Reason I'm asking is because I've got 6.5cm brass and 25.06 rem brass that is both new and I want to get the mv for both to put into my ballistic calculator for my binos but I only have the chrono for a limited amount of time so instead of chronoing now and again in maybe a couple of months time when I've got sized brass I can just do once and not worry too much about a big change.

[Leeman]

Depending what Chrono you use, you will likely see more variations due to atmospheric and ambient conditions than you will from any differences in brass.  As Brillo says, I've never observed any significant difference over a half dozen or so different cartridges either.

[Popsbengo]

19 hours ago, T3_tinker said:

Thanks guys. Reason I'm asking is because I've got 6.5cm brass and 25.06 rem brass that is both new and I want to get the mv for both to put into my ballistic calculator for my binos but I only have the chrono for a limited amount of time so instead of chronoing now and again in maybe a couple of months time when I've got sized brass I can just do once and not worry too much about a big change.

There are so many variables between load 'a' and load 'b' in your scenario that I wouldn't think your concerns are resolvable. For sure case volume will have a small effect but so does humidity of the powder (volume / weight variation), accurate depth of bullet seating, case neck tension, primer seating depth, etc. Not to mention batch differences for primers, bullets, powder. I tend to concentrate on making sure my S.D. is as small as I can get for a given batch and not worry about batch to batch variability (too much).

As Leeman says, weather effects will mask the small changes in case volume. Powder temperature is often ignored, I find this is a significant factor.

For peace of mind get a good zero grouping now and then check it's not moved significantly with the new batch later.

[Andrew]

FL resize your new brass and do the same on the second loading. This will help make the second loading as close to the first as you can make it.~Andrew

[VarmLR]

Agree with that. FL size at the start.  You'll probably want to re-chrono your load and adjust as necessary (for velocity) each powder batch and for significant changes in temperature anyway (keep notes for each load based on powder batch number and climatic conditions).

Just looking back through some 223 load notes, there was far less a difference (ie negligible) when FL sizing from the start over just firing new brass.

You'll want to do the same for any changes to primer but appreciate you're probably already aware.