Virgin brass vs final velocities......

[Koda]

.223 in an AR15...
I did an initial ladder test using brand new virgin brass, working my way up until pressure signs I stopped there (cracked case at 26.5g powder charge). I noticed a "plateau" in velocities on the upper end of charge weights and picked the lowest charge weight in the flat spot. From 5 rounds at 25.9grains of powder charge I was getting an average of 3051fps with a 55g bullet and a decent group. Cool.

So next range session I loaded 5 new virgin (0X) cases at the same 25.9g powder charge recipe. I also reloaded 15 of the once fired cases ...all from the same mfg purchase with the same recipe. Between the once fired (1X) brass and the virgin brass (0X) results were close enough to the same in accuracy and average velocities.. (1x: 2912fps, 0x: 2935fps). So basically between the two samples the average velocity was 2923.5fps.

Whats interesting is that new velocity is 127.5fps slower.
Its not that I really care to nitpick over ~128fps difference but my question is why? Is that difference because the virgin brass takes more energy to stretch during fireforming causing higher pressures? Ideally Id like to stay over 3000fps for other nitpicky reasons, (I dont think is unreasonable to ask for this bullet) but then why the virtually same loss with the 1x fired cases from the initial ladder test?

Notes: Since its a semi auto, I'm full length sizing the brass.

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[po18guy]

Ambient temperature.

 

[Koda]

Ambient temperature.

Im trying to remember to keep track of this, but using weather data history from the nearest town (which is actually fairly close to my spot...) the ladder test was 45degrees that day and then last weekends session was 54 degrees during the same timeframe.
My guess is that wouldnt make much difference?

 

[po18guy]

There is no other variable, is there? Same primers?

 

[DizzyJ]

What powder are you using?

 

[Koda]

There is no other variable, is there? Same primers?

There is no other variable, Same components, same powder charge... but I lost a tad over 100fps.

 

[Koda]

What powder are you using?

Accurate 2520. Its all I could find over the last year of looking for AR powder.

 

[Lennie]

Was the brass all from the same production lot? Within a brand of brass, production runs will have variances that will give different results.

 

[Koda]

Was the brass all from the same production lot? Within a brand of brass, production runs will have variances that will give different results.

yes, all was from the same box of brass.

 

[BigGame]

It is odd that the FF1 rounds were that much slower by 120+ fps. Usually it's the other way around. The 9 degrees different temperature wouldn't explain it. Is it possible that the humidity of the powder increased significantly between loadings? Did you, for example, leave your powder in the hopper rather than putting it back into the bottle right away? A significant increase in powder humidity will slow down velocity.

 

[Koda]

so I just had a thought, could it be from trimming the brass to case length?
But then on each session I was including a sample of new brass with the same results. Thats the odd part.
another thought is maybe the first time I shot faster resulting in slightly warmer barrel temp? I dont have a way to compare temps but IIRC I was needing to be home during the ladder test session that day but its not like a 20 round ladder test takes that long.

 

[BigGame]

Redirect Notice

 

[Koda]

It is odd that the FF1 rounds were that much slower by 120+ fps. Usually it's the other way around. The 9 degrees different temperature wouldn't explain it. Is it possible that the humidity of the powder increased significantly between loadings? Did you, for example, leave your powder in the hopper rather than putting it back into the bottle right away? A significant increase in powder humidity will slow down velocity.

Note: in the second range session included a sample of new unfired brass resulting in the same velocity loss as the reloaded brass sample. Thats the mystery to me.

Humidity is possible as well as case prep inconsistencies etc. I don't have a hopper so I put any back in the container and seal. I'm still learning reloading so I think any variables is hidden in my technique.... thinking about this, I think what I did different is on the second range session I decided to start full length sizing any unfired virgin brass for consistency. I dont think I sized the new brass on the ladder test.

Im guessing this will do it...?

 

[DizzyJ]

What are you using to check velocity?

 

[DizzyJ]

I'm also wondering if you have factory Ammo to run at the same time to see if it's velocity changes as well or stays the same.

Did you seat the bullets to the exact same length?

 

[Koda]

What are you using to check velocity?

A Caldwell optical chronograph.

I'm also wondering if you have factory Ammo to run at the same time to see if it's velocity changes as well or stays the same.

Did you seat the bullets to the exact same length?

Checking my notes. My first ladder test I seated to 2.260" saami spec. On my second test I seated to 2.255" because I noticed the bullet tips right on the surface of the mag wall. They functioned at 2.260" for 5rds but I felt .005" wouldnt hurt or make a great difference? Its possible it did but I would think it would increase velocity if anything. Its also possible I was inconsistent in my ladder test seating depth as I noticed a slight improvement in my standard deviations..... slight.

 

[DizzyJ]

A Caldwell optical chronograph.

This may very well be the issue.

Differences in lighting conditions and set up can cause a fair amount of variation in readings.

It's why a lot of people go with LabRadar or Magnetospeed.

 

[DizzyJ]

These are the same Bullets?

I doubt .005" is going to make that much difference unless you're jammed into the lands, and I doubt you were with 55gr fmj.

What manufacturer of bullets?

 

[Koda]

These are the same Bullets?

I doubt .005" is going to make that much difference unless you're jammed into the lands, and I doubt you were with 55gr fmj.

What manufacturer of bullets?

all the same bullets. Barnes 55g TTSX.

 

[Koda]

This may very well be the issue.

Differences in lighting conditions and set up can cause a fair amount of variation in readings.

It's why a lot of people go with LabRadar or Magnetospeed.

This I can believe. Ive had issues with errors with this chrono so much that I wont use it at my gun club anymore.
The last two range sessions have been on public lands for this reason, though both times it worked perfectly but this last time I intentionally moved it much closer to the muzzle cause I thought it would help reduce errors but I would think closer would read higher velocity not lower.
but maybe the outside sunlight was just different enough.

I cant afford a new chrono right now....

I might write this off as an anomaly and do a new finer tuned ladder test to get back up to 3000fps min.