FL vs NS sizing velocities.

[Canadian bushman]

In my 308 shooting a 215 berger my ns brass shot 2667 on avg compared to my fl sized brass shooting 2642.

Also had slightly different POI, with the slower load impacting higher.

Same load, same lot of brass, measured on the same scale and velocities checked on the same day with magnetospeed.

What exactly is causing my ns brass to produce higher velocities than my fl sized brass?

 

[pdog]

You potentially have tighter neck tension with neck sized brass verses the full length sized brass. Your full length die with the expander is possibly giving you a little less neck tension.
pdog

 

[Canadian bushman]

I use redding type s dies to size down and sinclair mandrels to final size all of my brass.

If it was a neck tension deviation it was because springback, not procedure.

 

[Down South]

In my 308 shooting a 215 berger my ns brass shot 2667 on avg compared to my fl sized brass shooting 2642.

Also had slightly different POI, with the slower load impacting higher.

Same load, same lot of brass, measured on the same scale and velocities checked on the same day with magnetospeed.

What exactly is causing my ns brass to produce higher velocities than my fl sized brass?

Could it be that the NS brass only is already fully fired formed to the chamber thus not loosing energy expanding the case body that is FL sized?

 

[Canadian bushman]

In my 308 shooting a 215 berger my ns brass shot 2667 on avg compared to my fl sized brass shooting 2642.

Also had slightly different POI, with the slower load impacting higher.

Same load, same lot of brass, measured on the same scale and velocities checked on the same day with magnetospeed.

What exactly is causing my ns brass to produce higher velocities than my fl sized brass?

Could it be that the NS brass only is already fully fired formed to the chamber thus not loosing energy expanding the case body that is FL sized?

This was my thought

 

[Canadian bushman]

What cycle were both on?
How much shoulder clearance were the FL? (headspaced / bumped)
How much closer force was needed to close the bolt on the NS?

The more clearance the FL have, the more pressure they will need to iron them to chamber walls.
Sort to speak, if you had a brass with 1/2-thou clearance verses one with 3-thou clearance, it will take more energy to iron/expand the 3-thousandths.

When ever a case is making contact with the chamber walls, pressure can spike up. The more/harder the contact, the more it can spike. I have pressure trace data that yield in excess of 3000-psi of pressure between minimal clearance brass to full contact (crush) fit brass. Inconsistent chamber contact of NS methods can create large ES do to these reasons.

My 2-cents
Donovan Moran

Both cases were on their 3rd firing
The fl sized had .002 clearance
The ns sized had moderate bolt resistance, being it was ns sized the 2nd time.
I usually have to shoulder bump after the 2nd ns sizing or chambering is difficult.

I think im gonna have to agree with down south and your reasoning

 

[Canadian bushman]

Wow donovan

Super informative and undeniable proof. Exactly what i was looking for. Thank you.

 

[Canadian bushman]

Donovan
Someone whos spent considerable time thinking about this, why do you think some folks fire forming rounds are so consistent (accurate)?

 

[Joe Salt]

I think you will find NOT ALL rifles like fire formed brass as they do formed! I know the 308 Baer likes fire formed brass as well as 300 WSM why is a good Question?

Joe Salt

 

[Canadian bushman]

Is there any posibility ,that since most fire forming involves forming a shoulder and jamming a bullet, the case is still able to maintain sufficient alignment and the required energy to blow out the shoulder acts to smooth the pressure curve yeilding in lower E.S./S.D.?

Just a wild thought

 

[Canadian bushman]

Thank you donovan for your time and sharing of your experience.
It is very much appreciated.

 

[Down South]

Donovan
Someone whos spent considerable time thinking about this, why do you think some folks fire forming rounds are so consistent (accurate)?

Good question..... and I have had exceptional accuracy come while fire-forming as well at times !.!.!
My opinion to it is two part: First, as long as they are consistent to each other dimensionally, they should produce consistent combustion cycles, regardless if they take more energy to adhere to the chamber walls.
Secondly, brand new brass is at its softest state it will ever be, so maybe since it hasn't been work-hardened any yet, it adheres more freely?
My 2-cents and theorem
Donovan

I share your thoughts on this as well.