Velocity shifts same load?

[Gilly]

This might be a stupid question but I have been noticing a trend recently on load development and then subsequent rounds sent by a chronograph. I have been handloading for 10 years+ not saying I am an expert, but not a newbie. As I chronograph more than I used to and gather more data this trend has emerged more frequently. It has happened on other cartridge's, bullets, powders and rifles, and a buddy has been seeing the same thing. Hopefully there are some here that can explain why it happens and how to mitigate it.

I am loading on an A&Dfx120 scale and calibrate it before use. I also have a bullet with a known weight sitting there to check occasionally during loading for consistency of what it shows for that bullet. The powder pan is also a good check for if scale drifts at all. So I feel like my powder drop is accurate to the resolution of the scale. I am especially picky on what I dump for load development vs. a match

What I see is shooting in the morning I am getting velocities somewhere between 30 & 50 fps slower than if I shoot in the afternoon. Same load, same seating depth etc. I can't imagine it is ammo temperature as in these cases I am loading up a ladder or later in load development larger strings in a temperature controlled environment, walking out the back door and shooting them within a few minutes, 5 or 10 max.

Example: 6 GT, Alpha brass, 112 match burner jumping .070, N555, Lab Radar for chronograph.
-Sunday afternoon: 46 degrees calm and sunny 38.1 grains was running 2875-2880 SD of 5
-Monday morning: 30 degrees light breeze 38.1 grains running 2830 with SD 3 and in the .3's
-Monday afternoon (same day): 42 degrees similar breeze, 38.1 grains back up to 2880. This time I ran 19 rounds over LR and had an SD of 8 which is expected with a larger sample size, first two groups were lower round count, lower SD may have been skewed.

I was shooting from same position with the lab radar set as consistently as I can get in relation to muzzle and target alignment. So is this an inconsistency in the Lab Radar from one session to another? If so why is it always faster in the afternoon? Ammo temperature seems unlikely where it is shot so quickly after loading. Externally is colder air and lower barometric pressure in the morning a cause? Just trying to figure out if it is something that can be eliminated, or just have to live with it? Maybe it is bad data and velocities aren't that far off, but then how can you trust a lab radar?

 

[mick243]

how stable is the temperature in your reloading room? as I understand it its mainly the starting temperature of the powder charge that affects its burn rate - I know people who take little warmer boxes to the range so that their ammo is always the same temperature no matter what the weather.

I would expect colder air to be more dense and therefore slow things down, but surely the distance between the muzzle and the chrono would be too small to have such an effect? otoh, lower baro pressure would be expexted to let things speed up a little - same problem with being too close to the chrony for effect though, and if you have one atmospheric speeding, but another atmospheric slowing, how much do they cancel each other?

one more possibility though - I wonder what temperature effects happen to the electronics in the chrono? maybe your ammo is working just fine but the readings on the machine are subject to temperature drift?

 

[Holliday]

how stable is the temperature in your reloading room? as I understand it its mainly the starting temperature of the powder charge that affects its burn rate -

I have never heard that one before…….

 

[Gilly]

how stable is the temperature in your reloading room? as I understand it its mainly the starting temperature of the powder charge that affects its burn rate - I know people who take little warmer boxes to the range so that their ammo is always the same temperature no matter what the weather.

I would expect colder air to be more dense and therefore slow things down, but surely the distance between the muzzle and the chrono would be too small to have such an effect? otoh, lower baro pressure would be expexted to let things speed up a little - same problem with being too close to the chrony for effect though, and if you have one atmospheric speeding, but another atmospheric slowing, how much do they cancel each other?

one more possibility though - I wonder what temperature effects happen to the electronics in the chrono? maybe your ammo is working just fine but the readings on the machine are subject to temperature drift?

Thanks for the ideas. Temperature is very stable. Room is in my basement with heat and ac controlled climate. I’d say it’s even more stable than the main level of our house. I don’t have a thermometer or anything there just speaking from feel. Might have to actually measure it to quantify temperature and humidity.

That was my thought on distance to chronograph that conditions shouldn’t be able to affect things as much as say ammo temperature which I’ve tried to control as much as possible.

 

[mtang45]

It might be atmospheric conditions as the Labradar Doppler reads reflection from the bullet base as it flies downrange, not as it flies past the Labradar near the muzzle. Labradar advertises +- .1% accuracy and at 2850 fps that only represents +-3 fps. However I'm sure that accuracy pertains to a single shot and not different sets of shots with differing conditions. I think its very likely that your projectiles are traveling the same speed (relatively) in the morning and afternoon and its the Labradar that is reading it differently. Just a guess, I could be way off base.

Best way to test to see if there is really a velocity difference is test at long range in the morning and then same target in the afternoon. See how much drop, if any, there is between groups.

 

[Doom]

Mtang45 brings up a potential issue with the Labrador that I’v never seen addressed. The the emissions from the unit form a cone in which the unit emits and receives the reflection of the bullet. I do not believe the unit reads actual muzzle velocity but picks up the velocity down range. To see 30fps difference that would be about 60 ft difference in flight.

I don’t know how sensitive the unit is to setup but it might be something you could test. I think the LR also can report velocity and distance. Might be worth looking into.

 

[larryh128]

This might be a stupid question but I have been noticing a trend recently on load development and then subsequent rounds sent by a chronograph. I have been handloading for 10 years+ not saying I am an expert, but not a newbie. As I chronograph more than I used to and gather more data this trend has emerged more frequently. It has happened on other cartridge's, bullets, powders and rifles, and a buddy has been seeing the same thing. Hopefully there are some here that can explain why it happens and how to mitigate it.

I am loading on an A&Dfx120 scale and calibrate it before use. I also have a bullet with a known weight sitting there to check occasionally during loading for consistency of what it shows for that bullet. The powder pan is also a good check for if scale drifts at all. So I feel like my powder drop is accurate to the resolution of the scale. I am especially picky on what I dump for load development vs. a match

What I see is shooting in the morning I am getting velocities somewhere between 30 & 50 fps slower than if I shoot in the afternoon. Same load, same seating depth etc. I can't imagine it is ammo temperature as in these cases I am loading up a ladder or later in load development larger strings in a temperature controlled environment, walking out the back door and shooting them within a few minutes, 5 or 10 max.

Example: 6 GT, Alpha brass, 112 match burner jumping .070, N555, Lab Radar for chronograph.
-Sunday afternoon: 46 degrees calm and sunny 38.1 grains was running 2875-2880 SD of 5
-Monday morning: 30 degrees light breeze 38.1 grains running 2830 with SD 3 and in the .3's
-Monday afternoon (same day): 42 degrees similar breeze, 38.1 grains back up to 2880. This time I ran 19 rounds over LR and had an SD of 8 which is expected with a larger sample size, first two groups were lower round count, lower SD may have been skewed.

I was shooting from same position with the lab radar set as consistently as I can get in relation to muzzle and target alignment. So is this an inconsistency in the Lab Radar from one session to another? If so why is it always faster in the afternoon? Ammo temperature seems unlikely where it is shot so quickly after loading. Externally is colder air and lower barometric pressure in the morning a cause? Just trying to figure out if it is something that can be eliminated, or just have to live with it? Maybe it is bad data and velocities aren't that far off, but then how can you trust a lab radar?

Eric Cortina covers this in one of his youtube videos. All about density of powder changing.

 

[Rockdoc173]

If you have a Kestrel, you could track atmospherics and apply those data points to each shot / velocity point and see if some kind of trend emerges while controlling as many variables as possible. I guess I look at it as loading related (your loading procedures, I don't think this is it though), component related (N555 is supposed to uber temp insensitive, but maybe not?), electronically related (problem with the chronograph), or atmospherically related (weather forces). Just start narrowing the list

 

[John Glidewell]

I had this happen in my GT and discussed it with Frank Green. Variations in powder, lot to lot and can to can - even if same lot number and/or different lots of bullets can produce that. I found my 2 lots of bullets I was using were just slightly different. .24325 and .24354. I measured many bullets multiple times. Does not seem like a lot but it is apparently enough to make a difference. Also, remember the lab radar is not 100%, it can have error, so you have to consider + - 5fps in that as well.

 

[RegionRat]

https://chronoplotter.com/2021/08/19/how-does-humidity-affect-powder/

 

[Holliday]

https://chronoplotter.com/2021/08/19/how-does-humidity-affect-powder/

Now this is something I am very interested in. It explains why my loads tend to be considerably hotter that Quick load predicts, leading to issues with proper case fill. Here in Wyoming we are a semi arrid climate. Very dry.

 

[RegionRat]

Start tracking the humidity of your powder, and of the space where you work, along with the velocity stats. You will see the correlation.

 

[Gilly]

Start tracking the humidity of your powder, and of the space where you work, along with the velocity stats. You will see the correlation.

Thanks. I am intrigued enough by the article shared by @RegionRat and a couple other comments I will be investing in some remote sensors to see if there is really that much shift in conditions in my room and in the powder containers. I think my room is very stable compared to where I used to reload, but may be wrong. Based on the average found in that paper in previous link I would need at least a 10% moisture content shift in my powder to happen in a 6-12 hour period to give me the 50+ fps shift. Not only that, but consistently move up and down by that amount, which isn’t totally inconceivable, but crazy to think about.

 

[max1840]

Thanks. I am intrigued enough by the article shared by @RegionRat and a couple other comments I will be investing in some remote sensors to see if there is really that much shift in conditions in my room and in the powder containers. I think my room is very stable compared to where I used to reload, but may be wrong. Based on the average found in that paper in previous link I would need at least a 10% moisture content shift in my powder to happen in a 6-12 hour period to give me the 50+ fps shift. Not only that, but consistently move up and down by that amount, which isn’t totally inconceivable, but crazy to think about.

I have a "SensorPush" unit in my reloading room. Has been working well for a number of years now.

 

[Holliday]

Thanks. I am intrigued enough by the article shared by @RegionRat and a couple other comments I will be investing in some remote sensors to see if there is really that much shift in conditions in my room and in the powder containers. I think my room is very stable compared to where I used to reload, but may be wrong. Based on the average found in that paper in previous link I would need at least a 10% moisture content shift in my powder to happen in a 6-12 hour period to give me the 50+ fps shift. Not only that, but consistently move up and down by that amount, which isn’t totally inconceivable, but crazy to think about.

I live in a cottage here in Wyoming with no AC. I am recently running a humidifier which keeps the place 30-40% RH for the winter. Powder is stored in a closet that runs 65°, to about 85° F on a hot summer day.

Now, someone here opined in another thread that these RH numbers are relative (hence the R), so I need to understand that point better. It seemed this person was saying “dewpoint” was a key indexer…..?

That said, I am looking at these for storing powder in a sealed container. : https://www.mtmcase-gard.com/mtm-ammunition-crate-12.php

My goal is to figure out an optimal storage humidity and somehow keep it controlled at that level inside the crate. Not unlike a cigar humidor. Then minimize the amount of time any powder is exposed to the cottage’s ambient RH% during the loading process. Any ammo stored long term could also go in a controlled RH controlled crate.

I am still working this out. I am wondering if a wood moisture reader might work to measure the actual moisture content in a batch of powder? Then of course the math to make that number correlate…..

 

[Doom]

I have refrained from commenting on this thread and in particular on the test that Region Rat posted. But I think its important to take the entire subject in perspective. My first comments are relative to the video and and the the test and its results. I will use relative humidity in lieu of dew point for consistency.

The author put together a rather good test. But what did the test conclude and how does it apply to powder storage and reloading? In test one for weight he puts his powder into a mason jar and seals it with humidity control. He allows the powder to be exposed to this environment of a total of 10 days (240) hours.

The author states:" Each sample took about 24-48 hours to stabilize to its target humidity, which it then maintained for the remainder of the time." It should be understood that both the control pack and the Kestral control and monitor the relative humidity of the air in the jar. It does not measure the moisture content of powder, so it is unknown as to when the powder reached equilibrium with the air, but it will be after the air has reached equilibrium with those devices. Keep in mind that the moisture in the powder in the bottom of the jar is not directly expose to air and must propagate from one kernel to the next to be exposed to the air in the jar. This is simply the way that mass transfer will happen. It is reasonable to assume that after ten days the air and powder are in or are close to equilibrium. Test one confirms that the powder is hygroscopic assuming that no other chemicals in the powder vaporize and are absorbed by the humidity control device. (I accept that the powder is hydroscopic). What the test does not answer is how rapidly the powder will absorb or release moisture to the air. When considering the individual kernel of powder, the hygroscopic mass transfer not only occurs at the air surface interface but between the the internal and external surfaces of the nitrocellulose. As a result the water must move from inside the kernel to the surface before it is lost to the air. or from the surface to the interior if the water is gained. This action takes time for equilibrium to be reached. Compare this to drying out a wadded washcloth.

The rest of the testing velocity testing confirms the original hypothesis that equilibrium moisture will affect burn rate and energy content per grain using velocity and pressure as surrogates.

What does the test not revel. First we must consider how the powder is stored prior to applying the results to our real world powder. First, in most cases the powder is stored in a plastic container. This container would not be considered to be hermetically sealed when leaving the factory, or at least the IMR, Hodgdon, Alliant or VV powders I have used would not be. However, when properly closed with the supplied seal the container does no breath or exchange air with the open environment readily. This assumption is easily verified by taking an 8LB empty powder container and placing it in a cold environment for a few hours, cap it tightly and then place it in a warm room. You will see the sides of the container bulge in a few hours as the air heats and expands. The container is actually holding air at a higher pressure than the surroundings.

Secondly, we need to consider how much moisture the air in the container might contain at any given time. if we assume the container is a 1.5 gallon container, then it could ultimately hold 1.5gallon/7.48 gallon/cubic foot of air or 0.2 cubic feet of wet air. Assuming we replace the air with 80F air at 95% relative humidity, that equals approximately 0.015 lbs of moist air and approximate 0.00026 lbs of water. 4 lbs of a single base powder (half full by weight) would have approximately 1.7% water at if in equilibrium at 60% RH per the authors data. That equates to 1.7/100*4 lbs or of water or 0.068 lbs or water. If all of the moisture in air of an empty container were to be absorbed the powder would now contain 1.706% water. Obviously this is an overestimate but is sufficient to demonstrate what will happen to the powder. That is not much. However it does demonstrate that each time the container is opened and the air inside is allowed to exchange it can and will have a very minor effect on the powder if different than equilibrium.

So what does this test demonstrate about what happens during the reloading process. Most of us will open the container, pour the powder into a hopper of some sort and proceed to load cartridges. First and foremost we by nature are probably reloading in a somewhat controlled environment with temperatures between 65 and 75 degrees and 40%-60% relative humidity. It is likely that our storage is in this range also. In this case if the powder started its life at 50% RH then very little to nothing will change over time or in the process of reloading since the driving force is small and the contact time is small. Further, most of the powder is not in direct contact with air for an extended period of time but is packed together with limited air exchange while in the hopper.

Now if you live in Tucson, or Anchorage, or Spokane, its likely that over time you are going to see your powder moisture change. It will occur gradually unless you are careless in your powder handling. It is not likely that you are going to see its characteristics measurably change during the reloading process.

 

[RegionRat]

First and foremost we by nature are probably reloading in a somewhat controlled environment with temperatures between 65 and 75 degrees and 40%-60% relative humidity

The point of sharing the link to the test report, is to point out that if you start tracking this instead of going on assumptions, you will in fact see the issue correlate to humidity where you are loading.

I used to think the conditions where I load at home were 40 - 60%, then I got surprised when I measured.

Water content does two things, it affects the density, and it also affects the ”burn rate”.

If you want to get very tight batch to batch velocity, best to start measuring the conditions where your powder is stored and where you load, or don’t but then don’t also act surprised when that shift hits. YMMV

 

[Gilly]

So after the last few days monitoring temperature and humidity in my reloading room using my kestrel I can say I confirmed my original suspicion that it’s a stable environment. Less than 4 degrees temperature fluctuation from morning, afternoon and evening. Humidity was also very stable at 40-42°. So I think it is most likely something with lab radar setup consistency or possibly another factor.

 

[Gohring65]

I had my mile dope go from 19 mils in the morning to 18 mils in the afternoon.
Temp was 40ish in the morning 65ish in the afternoon. I never really pursued it, I blamed it on temp.

 

[RegionRat]

I had my mile dope go from 19 mils in the morning to 18 mils in the afternoon.
Temp was 40ish in the morning 65ish in the afternoon. I never really pursued it, I blamed it on temp.

Yes, but...
There is internal ballistics, that deals with the muzzle velocity, and then there is external ballistics that deals with the trajectory.

The temperature of the ammo, gun, and weather between you and the target, is one place where the temperature issue overlaps.

Some of the above string was focused on the possibility that the alignment of the LR contributes MV measurement errors, and some of it was that the water content of the powder charge at the time of loading can contribute. The goal is to load with a consistent batch to batch velocity.

Both the humidity and the LR alignment must be tightly controlled to get a consistent MV. The dope is certainly affected by the weather, but the muzzle velocity changes force the dope to change for other reasons.

 

[Gohring65]

Yes, but...
There is internal ballistics, that deals with the muzzle velocity, and then there is external ballistics that deals with the trajectory.

The temperature of the ammo, gun, and weather between you and the target, is one place where the temperature issue overlaps.

Some of the above string was focused on the possibility that the alignment of the LR contributes MV measurement errors, and some of it was that the water content of the powder charge at the time of loading can contribute. The goal is to load with a consistent batch to batch velocity.

Both the humidity and the LR alignment must be tightly controlled to get a consistent MV. The dope is certainly affected by the weather, but the muzzle velocity changes force the dope to change for other reasons.

Yes I agree. I wasn’t really thinking when I sent that. I was actually at a stop light and read the op quickly. I forgot he said velocity changed and not dope.

 

[Kanwhitetails]

I've learned something here. I wouldn't have believed the moisture content within the RH of the surrounding room to effect powder anywhere near as much as it apparently might. Time to move the bench into a climate controlled environment...

Now, to explain to the wife why my man cave is migrating into the sewing room...

 

[mc10]

I have refrained from commenting on this thread and in particular on the test that Region Rat posted. But I think its important to take the entire subject in perspective. My first comments are relative to the video and and the the test and its results. I will use relative humidity in lieu of dew point for consistency.

The author put together a rather good test. But what did the test conclude and how does it apply to powder storage and reloading? In test one for weight he puts his powder into a mason jar and seals it with humidity control. He allows the powder to be exposed to this environment of a total of 10 days (240) hours.

The author states:" Each sample took about 24-48 hours to stabilize to its target humidity, which it then maintained for the remainder of the time." It should be understood that both the control pack and the Kestral control and monitor the relative humidity of the air in the jar. It does not measure the moisture content of powder, so it is unknown as to when the powder reached equilibrium with the air, but it will be after the air has reached equilibrium with those devices. Keep in mind that the moisture in the powder in the bottom of the jar is not directly expose to air and must propagate from one kernel to the next to be exposed to the air in the jar. This is simply the way that mass transfer will happen. It is reasonable to assume that after ten days the air and powder are in or are close to equilibrium. Test one confirms that the powder is hygroscopic assuming that no other chemicals in the powder vaporize and are absorbed by the humidity control device. (I accept that the powder is hydroscopic). What the test does not answer is how rapidly the powder will absorb or release moisture to the air. When considering the individual kernel of powder, the hygroscopic mass transfer not only occurs at the air surface interface but between the the internal and external surfaces of the nitrocellulose. As a result the water must move from inside the kernel to the surface before it is lost to the air. or from the surface to the interior if the water is gained. This action takes time for equilibrium to be reached. Compare this to drying out a wadded washcloth.

The rest of the testing velocity testing confirms the original hypothesis that equilibrium moisture will affect burn rate and energy content per grain using velocity and pressure as surrogates.

What does the test not revel. First we must consider how the powder is stored prior to applying the results to our real world powder. First, in most cases the powder is stored in a plastic container. This container would not be considered to be hermetically sealed when leaving the factory, or at least the IMR, Hodgdon, Alliant or VV powders I have used would not be. However, when properly closed with the supplied seal the container does no breath or exchange air with the open environment readily. This assumption is easily verified by taking an 8LB empty powder container and placing it in a cold environment for a few hours, cap it tightly and then place it in a warm room. You will see the sides of the container bulge in a few hours as the air heats and expands. The container is actually holding air at a higher pressure than the surroundings.

Secondly, we need to consider how much moisture the air in the container might contain at any given time. if we assume the container is a 1.5 gallon container, then it could ultimately hold 1.5gallon/7.48 gallon/cubic foot of air or 0.2 cubic feet of wet air. Assuming we replace the air with 80F air at 95% relative humidity, that equals approximately 0.015 lbs of moist air and approximate 0.00026 lbs of water. 4 lbs of a single base powder (half full by weight) would have approximately 1.7% water at if in equilibrium at 60% RH per the authors data. That equates to 1.7/100*4 lbs or of water or 0.068 lbs or water. If all of the moisture in air of an empty container were to be absorbed the powder would now contain 1.706% water. Obviously this is an overestimate but is sufficient to demonstrate what will happen to the powder. That is not much. However it does demonstrate that each time the container is opened and the air inside is allowed to exchange it can and will have a very minor effect on the powder if different than equilibrium.

So what does this test demonstrate about what happens during the reloading process. Most of us will open the container, pour the powder into a hopper of some sort and proceed to load cartridges. First and foremost we by nature are probably reloading in a somewhat controlled environment with temperatures between 65 and 75 degrees and 40%-60% relative humidity. It is likely that our storage is in this range also. In this case if the powder started its life at 50% RH then very little to nothing will change over time or in the process of reloading since the driving force is small and the contact time is small. Further, most of the powder is not in direct contact with air for an extended period of time but is packed together with limited air exchange while in the hopper.

Now if you live in Tucson, or Anchorage, or Spokane, its likely that over time you are going to see your powder moisture change. It will occur gradually unless you are careless in your powder handling. It is not likely that you are going to see its characteristics measurably change during the reloading process.

Well put. I'm currently running a long-term test monitoring humidity both inside a jug of H4350 (unsealed but with a tightened cap) and the room it's stored in. I'll publish the results likely late spring/summer, and should provide some data on how well powder containers prevent moisture transfer.

 

[Doom]

Sounds like a good idea! Hopefully you plan on leaving it closed for the whole time. If not and you open and close the container from time to time you see the air humidity changes in the jug make big changes due to air humidity.

 

[mc10]

Sounds like a good idea! Hopefully you plan on leaving it closed for the whole time. If not and you open and close the container from time to time you see the air humidity changes in the jug make big changes due to air humidity.

Yup, it's remaining closed the entire time. I've only opened it only once so far (in six months) to replace the Kestrel Drop battery, which I'll mark in the data

 

[secondofangle2]

https://chronoplotter.com/2021/08/19/how-does-humidity-affect-powder/

this is the most informative and stunning thing I have read in a whole long time

 

[Senor_Barney]

https://chronoplotter.com/2021/08/19/how-does-humidity-affect-powder/

So...


Should I go an open all 10 jugs of my Varget so they all acclimatize to rhe relative humidity of my garage at the same rate?

 

[RegionRat]

So...


Should I go an open all 10 jugs of my Varget so they all acclimatize to rhe relative humidity of my garage at the same rate?

No. The jugs are best left sealed tight and unopened, and stored as near to comfortable room conditions as possible till needed.

If you expose powder to really dry conditions, or really humid conditions, and let it normalize to those conditions, expect to see a velocity shift.

 

[straightshooter1]

No. The jugs are best left sealed tight and unopened, and stored as near to comfortable room conditions as possible till needed.

If you expose powder to really dry conditions, or really humid conditions, and let it normalize to those conditions, expect to see a velocity shift.

The "really dry" and the "really humid" conditions, that's really the big issue, huh?

For those who have a good controlled environment or live where humidity is around 40-50% all the time, it doesn't seem to me this is going to be an issue.

 

[RegionRat]

The "really dry" and the "really humid" conditions, that's really the big issue, huh?

For those who have a good controlled environment or live where humidity is around 40-50% all the time, it doesn't seem to me this is going to be an issue.

Agree, if the loading room humidity can stay between 40 - 50%, you will be okay.

Where I live, it is the difference between onshore wind, or desert wind (Santa Ana Winds). One gives us nearly perfect 50% and the other can drop us to below 15%.
With a marine layer (grey and foggy) the dew point comes close to the ambient temp and we can get up near 80% but it is easy enough to run a dehumidifier on those days and keep it nailed to 45 - 50%. YMMV

 

[Gtscotty]

Yup, it's remaining closed the entire time. I've only opened it only once so far (in six months) to replace the Kestrel Drop battery, which I'll mark in the data

Really interested to see your results. The info out thus far on humidity impacts to powder behavior are good, but left me wondering about real world humidity uptake of powder in opened then resealed jugs.