How to explain velocity decrease with lower COAL + more powder?

[rutilate]

I'm experiencing inexplicable velocity differences: shorter COAL and MORE powder = lower velocity.

I chronographed 9mm rounds in 2014 and found a combination of powder and COAL that gave me ~905fps. I shortened the COAL from 1.14 to 1.13 and redid the ladder test just to be sure. Now, it takes MORE powder to hit the same velocity.

That's 180 degrees from what I expected. How do you explain that?

The first loads I chronographed in 2014 in 2014 using a Caldwell Ballistic Precision chronograph (gen 1): 147gr X-treme bullets, 4.1gr Power Pistol, 1.14 COAL. Temp was 76 degrees. Average velocity is 904.5, Std dev = 15.3

Last week, I chronographed using a Gen 2(?) Caldwell Ballistic Precision chronograph (factory replaced due to failure of previous unit). COAL shortened to 1.13. The same charge of 4.1gr Power Pistol yielded an average velocity of 851fps. To achieve the same velocity I had to increase the charge 4.4gr Power Pistol. Temp was 70 here. Avg velocity = 905.1, std dev=17.7;

The primary differences are COAL, 6 degrees cooler, a different part of the country, and a new model of the same chronograph.
Shortened COAL SHOULD have increased velocity. 6 degrees cooler shouldn't have made a difference. Powder is about 6 mos old, stored in the original container.
Could the different model of the same chronograph make that much of a difference?

Which chronograph measurement do I trust?

How do you explain this?

Read More

 

[No_Regerts]

Was there any direct sunlight on the chrono in either instance?

 

[bbbass]

The most likely explanation is the diff in chronos. If you can borrow a friends, you might see a diff between the two. Not sure if there is a way to calibrate chronos...

Another possiblity is old powder. If you are using the same can of powder that you used in 2014 it has deteriorated.

 

[No_Regerts]

One thing I found with mine is if there is uneven ground where somehow the distance between the fields was lengthened or shortened by how the unit was situated, ill get different readings.

 

[rutilate]

Was there any direct sunlight on the chrono in either instance?

The earlier version of the chrono had some challenges with the light/LED combo, but the second time around it was overcast and they've reworked the light/LED combo to provide more shade.

The most likely explanation is the diff in chronos. If you can borrow a friends, you might see a diff between the two. Not sure if there is a way to calibrate chronos...
Another possiblity is old powder. If you are using the same can of powder that you used in 2014 it has deteriorated.

New powder--about 4 months old. Definitely need to visit the range with a friend's chronograph.

 

[KKG]

Build some reloads using the old days and see what velocity you get with the New chorograph.

 

[gmerkt]

The primary differences are COAL, 6 degrees cooler, a different part of the country, and a new model of the same chronograph.

Where did you move from? Altitude will vary performance. At higher altitude, air is less dense, easier for a bullet to pass through it. This would result in a higher velocity.

Too bad you had to change chrono. This makes it more difficult to figure out what's going on. The COAL difference is slight, probably not a factor. I'd think if anything, shortening it would increase pressure (and velocity) in 9mm, Temp. difference of 6 degrees probably not a factor. How about humidity? Humid air is more dense with moisture, will slow the bullet down.

Primer brand/type hasn't been mentioned as a variable.

You said your first data was from 2014. Your second data from last week resulted from using powder that was "about 6 months old." So this information suggests that four and a half years ago you used different powder of the same type. But not necessarily the same lot. Could make a difference.

Could the different model of the same chronograph make that much of a difference?

Very definitely. Particularly in this case, as you had to send back chrono #1 as defective. My own view is that consumer grade chronos are like consumer grade thermometers. They aren't necessarily scientifically precise but give good approximations. Easiest thing to do is run your round across someone else's chrono, see if you get closer to 850 or 900.

There is a science to this. When we read posted velocities in reloading manuals, those have been established under controlled conditions. Small arms ballistics testing is done inside indoor ranges where they can control the environment. They use consistent batches of the same components. And of course the same instrumentation from test to test. Most of us who reload privately don't have access to these assets so we have to do with approximations. We can narrow things down a bit by attempting to standardize as much as we can.

 

[v0lcom13sn0w]

ive always found it odd that my 308 win load should yield 2650fps out of a 24" barrel according to nosler. ive repeatedly got 2800 fps with a 20" barrel with 3 different chronos.

not complaining though. just odd.

 

[techiej]

I'm experiencing inexplicable velocity differences: shorter COAL and MORE powder = lower velocity.

I chronographed 9mm rounds in 2014 and found a combination of powder and COAL that gave me ~905fps. I shortened the COAL from 1.14 to 1.13 and redid the ladder test just to be sure. Now, it takes MORE powder to hit the same velocity.

That's 180 degrees from what I expected. How do you explain that?

The first loads I chronographed in 2014 in 2014 using a Caldwell Ballistic Precision chronograph (gen 1): 147gr X-treme bullets, 4.1gr Power Pistol, 1.14 COAL. Temp was 76 degrees. Average velocity is 904.5, Std dev = 15.3

Last week, I chronographed using a Gen 2(?) Caldwell Ballistic Precision chronograph (factory replaced due to failure of previous unit). COAL shortened to 1.13. The same charge of 4.1gr Power Pistol yielded an average velocity of 851fps. To achieve the same velocity I had to increase the charge 4.4gr Power Pistol. Temp was 70 here. Avg velocity = 905.1, std dev=17.7;

The primary differences are COAL, 6 degrees cooler, a different part of the country, and a new model of the same chronograph.
Shortened COAL SHOULD have increased velocity. 6 degrees cooler shouldn't have made a difference. Powder is about 6 mos old, stored in the original container.
Could the different model of the same chronograph make that much of a difference?

Which chronograph measurement do I trust?

How do you explain this?

The difference in velocity is too small to worry about - 904.5 vs 905.1, or 0.6 fps which if my math is right is a difference of 0.067%. Even if you are absolutely precise in all of your other measurements, you could easily have this much variance just between different brands of brass due to case wall thickness, if one was stepped, etc., variance in bullet weights (even from the same batch, loading variances (especially if using a progressive press), etc.

Aside from everything else, how many shots made up each test and what was the extreme spread in each group?

 

[gmerkt]

ive always found it odd that my 308 win load should yield 2650fps out of a 24" barrel according to nosler. ive repeatedly got 2800 fps with a 20" barrel with 3 different chronos.

Tail wind?

Seriously, I was shooting next to a guy in Carson City one day. He couldn't understand why his .222 Rem. with 40 gr. bullets was printing an inch to the right. He said, "I can't understand it, just last week I was hitting right in the center of the target." I suggested to him that it might be the approx. 15 mph crosswind we were experiencing that day. After all, a 40 gr. bullet ain't much.

Since you've checked it on three diff. chronos, you might ask Nosler about this. It could be anything up to and including an error in the book, after all, everyone makes mistakes. A typo on velocity isn't too big a deal, hopefully they copy edit charge weights pretty carefully before publishing.

 

[gmerkt]

The difference in velocity is too small to worry about - 904.5 vs 905.1, or

Just to be clear, velocity readings from his second test were 851 fps. using the same load from his first test. To get 905.1, he had to bump his charge to 4.4 gr.

 

[bbbass]

The difference in velocity is too small to worry about - 904.5 vs 905.1, or 0.6 fps which if my math is right is a difference of 0.067%.

Except for this:

Average velocity is 904.5,

The same charge of 4.1gr Power Pistol yielded an average velocity of

851fps

Unless my calculator is broken, that is a diff of 53.5 not 0.6fps.... you see it differently???

Edit: posted this while gmerkt was posting, didn't intend to hammer away on it. Also my effort to highlight the fps in a different color is resulting in a separate quote... weird

 

[techiej]

Except for this:

Unless my calculator is broken, that is a diff of 53.5 not 0.6fps.... you see it differently???
[/QUOTE]
You're correct - I was looking at the other velocities listed; which would make it about a 6% difference.

 

[No_Regerts]

ive always found it odd that my 308 win load should yield 2650fps out of a 24" barrel according to nosler. ive repeatedly got 2800 fps with a 20" barrel with 3 different chronos.

not complaining though. just odd.

Its the Schwartz

 

[bbbass]

Unless my calculator is broken, that is a diff of 53.5 not 0.6fps.... you see it differently???

You're correct - I was looking at the other velocities listed; which would make it about a 6% difference.
[/QUOTE]

6% doesn't seem like a lot, but the OP has presented an interesting problem.

 

[bbbass]

Its the Schwartz

Schwartz's cat?
Schwartzennegger??

 

[No_Regerts]

Schwartz's cat?
Schwartzennegger??

 

[bbbass]

View attachment 615445

Dang, I thought you said "Schrantz"!

 

[DizzyJ]

I'd chalk it up to the difference in chronographs, and or different powder lots.

I sold my chrony because I didn't trust it. I use a Magnetospeed and I feel a lot more confident with it. If you know someone with a Labradar, I'd also feel confident with that as well.

 

[rutilate]

Thanks for the comments, all. You've given me a couple of variables to test.

I'm going to recreate the old loads and test with the new chronograph to double-check the baseline. And I have a friend with a Labradar and we'll see how well the chronograph is functioning.