Velocity change, humidity the culprit?

[engineerjet]

So I loaded some 40 and did a chrono.

 

Specs and Results:

40 sw 180gr Xtreme plated

4.6 VV N320

Caldwell ballistic precision chrono with leds under the sunshade to get a reading indoors.

Shooting out of a 5.4 barrel

Consistent 1200 FPS (according to the chrono, seemed high)

 

A week later I tested another batch, made some using 4.6 gr as a reference and it shot average of 800 FPS. I couldnt for the life of me think of what had changed. I realized that I had left the powder in the powder feed with the hopper lid off for a week. Could that affect the powder that much to result in a drop of 400 FPS? Its the only thing I can think of that is different. Has anyone ever experienced something similar?

 

[Furrly]

Chrono issue 

[Sinister4]

Yup I always test a factory loaded known velocity round before and after each chrono setup to make sure all is well.  If its way off check everything

[RePete]

I loaded 4.7gr/N320 under a 180gr Frontier and my velocities were 978 fps from an STI Trojan and 993 fps from a G35.

 

Humidity does affect the velocity.  

 

I was the chronographer for a major match and one of the shooters liked to play close to the line (165000) and passed at one match and 2 weeks later, using the same gun and the same batch of ammo, failed chrono at 162000.  We had a serious rain storm go through on the Saturday and he shot on Sunday.  Temps were 95 degrees and the humidity was about 90 or so %.

 

I say that you have a chrono problem too.

[Farmer]

If I’m reading you right your thinking that your Powder picked up moisture sitting in the hopper and that’s the reason for your velocity drop? As others have stated their loads with similar charges seem to be closer to what you’re second test was. Was your chrono set up the same the second time? Lighting in the area the same? (Even with LED’s) Conditions make a difference but that’s a lot. 

[engineerjet]

49 minutes ago, Farmer said:

If I’m reading you right your thinking that your Powder picked up moisture sitting in the hopper and that’s the reason for your velocity drop? As others have stated their loads with similar charges seem to be closer to what you’re second test was. Was your chrono set up the same the second time? Lighting in the area the same? (Even with LED’s) Conditions make a difference but that’s a lot. 

I was in an indoor range and in different ports so the lighting was different. Next time i'll try a port that is poorly lit so it doesnt interfere with the sensors.

[RePete]

If the range is light by florescent lights then you are wasting your time.  The flickering (frequency) interferes with the sensors on the chrono.

[Postal Bob]

4 hours ago, RePete said:

If the range is light by florescent lights then you are wasting your time.  The flickering (frequency) interferes with the sensors on the chrono.

^This, Fluorescent lights mess up everything. Plus, I never had good results with the chrono using plated bullets. Not sure, but it was suggested that they're very shiney compared to regular jacketed bullets. And that it plays havoc with the sensors.

[zzt]

Caldwell chronos are basically crap.  Even the upside down one has issues.  ANY light that falls directly onto the measuring ports screws up results.  I went back to my Pro Digital Chrono and have no issues.  The Caldwell ended up in the trash (both of them).