Velocity DECREASES 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 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 to 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?

[usrifle]

Do you still have the older Chrono, or did Steve shoot it? Run them across that and see if there's a difference.

Otherwise, they went Bang...cycled the Pistol and shot to POI? I wouldn't get all hung up on it.

[Rutilate]

Do you still have the older Chrono, or did Steve shoot it? Run them across that and see if there's a difference.

Hah! That's exactly the problem. Let's all blame Steve!

The older chrono started failing with ERR2, a known problem, so I'm not sure I could get it to work long enough to create a test.

What I probably should do is offer Steve a really good bag of coffee beans as an incentive to bring his fancy chronograph to the range.

And you're right, it does work, but I'd like my reloading to be a bit more deterministic, especially if I'm allowing some of our ladies to shoot our specialty ammo (like the .460 S&W Magnum with a reduced power load).

[beckdw]

You said the original load was developed in a different part of the country.

Was there a significant elevation and or barometric pressure difference between the two locations? Less air pressure when you developed the original load vs more pressure here could absolutely slow the velocity that much. My 300 blackout loads can have ~100fps difference shooting east or west side of the state.

I'm assuming different lots of powder? Could make enough of a difference in theory, that's why they tell you to develop your load again when you change lots.

[MadPick]

Otherwise, they went Bang...cycled the Pistol and shot to POI? I wouldn't get all hung up on it.

Agreed.

But yes, I could be bribed to bring the Labradar out to the range ... if you let me shoot the Caldwell. Errr, I mean over the Caldwell.

[ironworker78]

Same gun?

[Rutilate]

Same gun?

Yes, actually.

You said the original load was developed in a different part of the country.
Was there a significant elevation and or barometric pressure difference between the two locations? Less air pressure when you developed the original load vs more pressure here could absolutely slow the velocity that much. My 300 blackout loads can have ~100fps difference shooting east or west side of the state.
I'm assuming different lots of powder? Could make enough of a difference in theory, that's why they tell you to develop your load again when you change lots.

Earlier was Boston so about the same elevation, but it was certainly much more humid when I did the first test. No way of knowing beyond that. I never would've dreamed that you'd get that much velocity variation.

And certainly different powder lots. Again, wouldn't have expected a significant variation in powder, but at this point anything is possible!


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

Thanks, all!

[Old Growth]

Too many variables. Your not comparing apples to apples.

Different locations.
Different measuring tools.
Different powder.
Different weather/humidity.


You need to load some at your original load recipe and length and test them on the same day as and same chrono as your new loads. This will give you ACCURATE data.

Until then your results are not that comparable. IMO.

[hartcreek]

Yep to many variables......I have a 8 pound bottle of Unique that I have been using mostly for .44 Special and that bottle is so hot that I have had to back down grains clear down to 5 with a 240 grain bullet to be able to extract from my 85 624. I had polished the machine marks out and it made no difference.

[No1Uno]

i have 2 caldwells of the same model, but different years. about 2 years apart. i set them up inline with each other to test to see how close they would be. i also had a labradar set up to read and lined that up too.

all three read a little different on every shot. usually they were all within 10-15 fps of each other, but an occasional larger spread. really confused me and not sure which one to believe. and anyone know how chronos can be tested for accuracy?

[Rutilate]

i have 2 caldwells of the same model, but different years. about 2 years apart. i set them up inline with each other to test to see how close they would be. i also had a labradar set up to read and lined that up too.

all three read a little different on every shot. usually they were all within 10-15 fps of each other, but an occasional larger spread. really confused me and not sure which one to believe. and anyone know how chronos can be tested for accuracy?

That’s a great question!
I know the Caldwell iphone app asks for distance from muzzle. I ASSUME that it will adjust velocities based on the value in that field, but I haven't explored it further.

I expect the Labradar is going to be the more accurate of the three. Higher price = better, right?

Competition shooters told me years ago they always planned on their velocities being about 10% over the power factor threshold to account for differences in chronograph readings at the various matches they went to.

https://www.longrangehunting.com/threads/how-to-check-a-chronograph.80435/
Comment #4 here talks about how they found one chronograph to be inaccurate over another.

Much more important at longer range, but it appears that 150fps variation between different models of chronograph is not unheard of.