How does humidity affect powder?

Table of contents:

• Why is it important?
• Background information
• Questions to answer
• Weight testing
• Live fire testing
• Closing thoughts

You may have heard about a relationship between humidity and bullet velocity either in a book, on a reloading forum, or from a crusty benchrest shooter right after saying they get better ES/SD’s by only using Poland Springs in their wet tumbler.

Humidity is largely discussed as it relates to calculating ballistic solutions, but one relatively unexplored aspect of humidity is the direct effect it has on your powder itself.

Smokeless powder is hygroscopic – that is, it has an affinity for absorbing moisture from the air. Powder can be expected to perform differently if it’s wetter or dryer, which is why precision reloaders have long-recommended storing components at consistent conditions, but little public research has been done to put actual numbers to it.

I stumbled on the topic of powder humidity almost by accident. In June I flew to Wyoming to compete in Nightforce ELR, where I learned during my Applied Ballistics PDM that my 6.5 Creedmoor load was clocking in a full 54 fps slower than I expected. I chronographed my load at 2,786 fps some months prior with a LabRadar, and a change of this degree was shocking. My stock Tikka barrel wasn’t nearing the end of its life yet with only ~1,400 rounds on it, and while Wyoming’s environmentals are different from Massachusetts (hotter, dryer, higher density altitude) those differences should lend to the load being faster. I wondered if the plane trip somehow had an effect on the ammo, or if maybe I hadn’t fouled my barrel properly leading up to the PDM. After returning home however I loaded a fresh batch of rounds and confirmed this was, in fact, my new velocity and not just a fluke.

After my PDM I had a chance to talk to Bryan Litz about the discrepancy, and he immediately suggested powder humidity could’ve played a role.

Why is it important?

To put it simply:

“[O]nce you break the seal on a jug, that powder starts to acclimate to whatever your environmental, ambient humidity is. …That’ll change a great deal the way that it burns.”

Bryan Litz, Powder Humidity / Temperature Storage

In your reloading you’ve probably come across a smokeless powder burn chart such as this one. While there are a number of ways to characterize powder performance, powders can be generalized and quickly compared according to their overall burn rate. Hodgdon Titewad (pistol powder) burns faster than H4350 (rifle powder) which burns faster than Retumbo (magnum rifle powder).

Powder manufacturers ship their products targeting a specific moisture content (according to the product’s formulation). But the moment the seal is broken that moisture content begins to change. If you live in a region with a year-round humidity similar to factory levels (typically 40-55% RH), or if the conditions in your reloading room are tightly controlled, there may be minimal change. Here in New England however, the ambient humidity swings wildly with the seasons.

Background information

An article simply named “Powder” (alternatively named “All about powder and a little more”) by Sven-Eric Johansson, published in the Norma Reloading Manual: Expanded Edition, is likely the most comprehensive and authoritative research publicly available on the topic. In the article Sven-Eric details the chemical composition, manufacturing process, and numerous other factors that influence the performance of smokeless powders. An entire section is devoted to moisture content – why powder absorbs moisture, the relationship between relative humidity and moisture content, and how it affects the burn rate of various Norma powders. If you don’t already own it, I highly recommend picking up a copy and reading the article for yourself.

The main ingredient in smokeless powder is nitrocellulose, a substance formed by subjecting cellulose (a fibrous material found in plant cell walls) to a complex set of chemical and manufacturing steps.

On page 112 of the manual, Sven-Eric explains:

“[N]itrocellulose is hygroscopic – this means it has the ability to absorb a certain amount of moisture from the surrounding environment; the amount of moisture is related to the relative humidity.

“The moisture content of a powder directly affects the burn rate of the powder – a powder with low moisture content will burn faster than a powder with high moisture content.”

While the exact composition of H4350 is not publicly known, we know it’s a single base powder which means it’s based solely on nitrocellulose. (Double base powders rely on both nitrocellulose and an added percentage of nitroglycerin for propulsion.) Single base powders are commonly 90-98% nitrocellulose by composition.

Adding other substances to the composition like nitroglycerin can reduce a powder’s sensitivity to humidity, making single base powders more sensitive to humidity change than their double base counterparts:

Sven-Eric went on to further measure humidity’s effect on velocity and pressure for a number of Norma powders:

Important takeaways from this testing are that: 1) humidity has a sizable effect on powder performance, and 2) this effect is not perfectly linear.

Powder does not absorb moisture at a constant rate as it’s subjected to different humidities, and these rates can change according to the exact composition of the powder itself. It’s unclear in the article if only two data points were collected for velocity testing, but a clear pattern was observed in humidity having an increased effect on pressure towards the extreme ends of the scale (lower than 40%, higher than 70% RH).

Since the release of this Norma article, little has been published on the topic that can be considered comprehensive.

I wanted stronger data about humidity’s effect on my own personal reloading. How consistent should my storage conditions be reasonably kept? Exactly what kind of effect can humidity have on my powder’s performance? How much can my load be expected to change when conditions change?

The big questions

I chose to test H4350 since I personally use it in my PRS load. To understand humidity’s effect, we actually need to break it down into two separate questions:

• How much does H4350 hygroscopically absorb moisture, and to what degree does it affect charge weight?
• How does moisture absorbed in H4350 affect the powder’s burn performance?

Changing the moisture content of powder can affect a load in several potential ways. Powder with a higher moisture content contains proportionally less propellant by weight, so it stands to reason that 41.50 gr of H4350 conditioned at 90% RH will produce less overall energy and slower velocities than the exact same charge weight at 10% RH.

It’s easy to condition powder to different humidity levels and compare velocities, but a test like this conflates multiple variables and doesn’t provide the full picture we’re looking for. Increased moisture not only makes powder kernels heavier, but the added water also absorbs more energy from the explosion. Without isolating these effects from one another, it’s impossible to tell how much of our velocity change is caused by one or the other.

To address this, I decided to run two experiments in parallel.

Experiment 1

The first experiment reflects the “real life scenario” and shines light on question #1. Say you’ve unsealed a jug of H4350 and over time the jug conditions to a different ambient humidity in the room. How does this affect your charge weight, and how much can your load’s behavior be expected to change overall?

I began by measuring five equal portions of 600.00 gr H4350, weighed on a calibrated, power conditioned, grounded FX-120i scale. Each portion was quickly poured into an airtight Mason jar with a cigar humidor pack, along with a Kestrel DROP D2 to wirelessly monitor the relative humidity inside each jar.

Boveda humidor packs provide two-way humidity control and maintain a specified target % RH within their containers. I used these packs to condition my powder samples – a technique also used by Applied Ballistics in their own testing.

The five jars contained:

1. 600.00 gr H4350 (control)
2. 600.00 gr H4350 + desiccant pack
3. 600.00 gr H4350 + 32% RH humidor pack
4. 600.00 gr H4350 + 65% RH humidor pack
5. 600.00 gr H4350 + 84% RH humidor pack

Each jar was cranked tight, labelled, and further sealed in a Ziploc bag.

Temperature and relative humidity were recorded for each sample every 10 minutes by the Kestrel DROPs, which connected to the Kestrel LiNK app via Bluetooth. The samples were stored in my reloading room away from sunlight for a total of 10 days.

Each sample took about 24-48 hours to stabilize to its target humidity, which it then maintained for the remainder of the time.

Additionally, immediately upon unsealing the H4350 jug, a Kestrel DROP was temporarily placed inside the jug itself and measured a factory humidity level of 51.0% RH. Since the manufacturer jug is not hermetically sealed (the reason for this whole test in the first place!) I created a separate “baseline” sample to act as a control for the experiment. This baseline sample contained only H4350 and no humidor pack or desiccant. A minor change was recorded as the sample equalized inside its container around 52-53% RH.

Initial measurements for Experiment 1

At the end of 10 days, the samples were measured:

Sample	Relative Humidity	Total Weight	Weight Change
Control	53.0% RH	599.88 gr	Baseline
Desiccant	14.7% RH	596.22 gr	-3.66 gr (-0.61%)
32% humidor pack	34.9% RH	598.16 gr	-1.72 gr (-0.29%)
65% humidor pack	66.5% RH	601.34 gr	1.46 gr (+0.24%)
84% humidor pack	84.0% RH	604.68 gr	4.80 gr (+0.80%)

These differences seem small at first glance, but placing them in context is where the real story is. These numbers suggest that a 41.50 gr charge dispensed from a H4350 jug conditioned at 34.9% RH would, in actuality, contain the equivalent amount of propellant as a 41.62 gr charge at factory conditions. A 41.50 gr charge dispensed from a jug conditioned at 14.7% RH (say, through the dead of winter) would contain the equivalent amount of propellant as 41.75 gr conditioned at 53.0% RH. Not even factoring the double effect of dryer powder having an increased burn rate as well.

It’s important to note these numbers only represent this specific lot of H4350 I tested. Other lots may contain different formulations or factory moisture levels that decrease or increase this effect.

Experiment 2

The second experiment hones in on question #2 and isolates moisture content as the sole variable. I hoped to answer exactly how much of the change in powder performance was due to moisture alone, rather than a combination of factors.

I followed a similar process to the first experiment but with one important difference. Rather than conditioning bulk amounts of powder, I began by measuring exactly 12 rounds’ worth of powder (41.50 x 12 = 498.00 gr) at factory humidity level. This sample was poured into an airtight Mason jar with a Boveda pack and Kestrel DROP D2, and was repeated for each humidity level tested.

The five jars contained:

1. 498.00 gr H4350 (control)
2. 498.00 gr H4350 + desiccant pack
3. 498.00 gr H4350 + 32% RH humidor pack
4. 498.00 gr H4350 + 65% RH humidor pack
5. 498.00 gr H4350 + 84% RH humidor pack

After conditioning, each sample would then be equally divided into 12 rounds regardless of the sample’s new weight. Measuring samples in this way allows us to directly compare the same initial charge weight at different humidities. A sample at 32% RH will contain exactly the same amount of propellant as an 84% RH sample, with moisture content being the only variable.

Initial measurements for Experiment 2

At the end of 10 days, the samples were measured:

Sample	Relative Humidity	Total Weight	Weight Change
Control	52.2% RH	497.92 gr	Baseline
Desiccant	16.9% RH	494.72 gr	-3.20 gr (-0.64%)
32% humidor pack	34.9% RH	496.46 gr	-1.46 gr (-0.29%)
65% humidor pack	66.1% RH	499.16 gr	1.24 gr (+0.25%)
84% humidor pack	82.3% RH	501.82 gr	3.90 gr (+0.78%)

These weight changes (expectedly) line up almost exactly with the ones recorded in Experiment 1.

Performing a linear regression and calculating the y-intercept lets us estimate that the total weight of a completely desiccated sample containing no moisture would be 492.76 gr. This is a 5.16 gr difference from our 497.92 gr baseline, which lets us calculate that H4350 has an approximate 1.04% moisture content. This falls squarely within typical range according to the Norma powder article.

Loading test cartridges

Experiment 1

Twelve rounds from each of the five bulk powder samples (60 in total) were loaded with the following recipe:

• Powder: 41.50 gr H4350
• Brass: 3x fired Lapua, 6.5 Creedmoor LRP
• Primer: CCI BR-2
• Bullet: 6.5mm Berger 130 gr VLD Target

I wet tumbled the cases, resized with a 6.5 Creedmoor Redding Type S Bushing FL Sizing Die with 0.288″ bushing, shoulder bumped 0.0020″, neck expanded with a 21st Century 0.2625″ mandrel, trickled to the kernel with an FX-120i scale, and seated with a 6.5 Creedmoor Short Action Customs Seating Die to a factory-recommended 2.800″ COAL. Loading was performed on a Forster Co-Ax press. Each powder sample was opened for the minimum amount of time necessary, and I worked to accurately measure, charge, and seat each cartridge as quickly as possible.

Experiment 2

After weighing, each sample was carefully divided into 12 equal portions for a total of 60 rounds:

Sample	Total Weight	Charge Weight
Control	497.76 gr (497.92)	41.48 gr
Desiccant	494.64 gr (494.72)	41.22 gr
32% humidor pack	496.32 gr (496.46)	41.36 gr
65% humidor pack	498.96 gr (499.16)	41.58 gr
84% humidor pack	501.60 gr (501.82)	41.80 gr

and loaded with the same recipe and process as Experiment 1:

• Brass: 3x fired Lapua, 6.5 Creedmoor LRP
• Primer: CCI BR-2
• Bullet: 6.5mm Berger 130 gr VLD Target

Since the FX-120i has a resolution of 0.02 gr (approximately a single H4350 kernel), and I wasn’t about to start cutting individual kernels, a small number of kernels were leftover after dividing up each sample. In these cases, the new smaller total weights were recorded and a note was made to scale (by a very minor amount) any necessary calculations to account for the change.

With this second batch of cartridges ready, it was time to shoot some dollar bills into a berm.

Live fire testing

I tested using my Tikka T3x CTR with a 24″ factory barrel. The weather at the range was 77°F 62.6% RH with a 1,864 DA according to my Kestrel.

For data collection, I recorded velocities with a LabRadar and chamber pressure with a PressureTrace II.

The PressureTrace system is based on a strain gage installed directly above the chamber. The gage links to a control box, which transmits data via Bluetooth to a PC running software that records the pressure curves.

I began with 10 fouling rounds and ensured everything was recording properly.

Once the barrel cooled down, I then shot the 12 round control group (unconditioned 41.50 gr H4350) from Experiment 1.

To ensure consistency between shot strings, I monitored the temperature of the chamber with a FLIR camera and waited until it reached 90-100°F before continuing testing (approx. 5 minutes).

It’s important to note here that reading #2 (which is immediately adjacent to reading #1) in the FLIR image isn’t actually 81.5°F. Depending on composition, steel barrels can have a relatively low emissivity level which results in inaccurate temperature readings. One way to account for this is to measure a higher emissivity material like electrical tape (or the plastic tape of the strain gage) on the target.

For each string of 12 shots, the PressureTrace II recorded and displayed a pressure curve chart like the ones below. The software limits 10 shots per file, so each string was split across two files.

Live fire results

Experiment 1

Muzzle velocity:

Group Tested	# of Samples	Stats	Avg. Velocity
52.7% RH (Control)	n=11	ES 25, SD 8.1	2,786 fps
14.5% RH (Desiccant)	n=10	ES 54, SD 15.9	2,879 fps
34.8% RH (32% humidor pack)	n=12	ES 29, SD 10.8	2,838 fps
66.5% RH (65% humidor pack)	n=12	ES 50, SD 12.4	2,747 fps
83.5% RH (84% humidor pack)	n=12	ES 31, SD 9.2	2,650 fps

Chamber pressure:

Group Tested	# of Samples	Avg. Pressure	Peak Pressure
52.7% RH (Control)	n=8	51,259 PSI	52,376 PSI
14.5% RH (Desiccant)	n=10	54,396 PSI	55,027 PSI
34.8% RH (32% humidor pack)	n=12	52,917 PSI	54,000 PSI
66.5% RH (65% humidor pack)	n=12	50,157 PSI	50,711 PSI
83.5% RH (84% humidor pack)	n=12	48,146 PSI	48,921 PSI

Unfortunately one cartridge in the Control group misfired, and two shots in the Desiccant group weren’t recorded on the LabRadar due to acquisition and user errors.

Experiment 2

Muzzle velocity:

Group Tested	# of Samples	Stats	Avg. Velocity
52.2% RH (Control)	n=12	ES 45, SD 12.2	2,790 fps
16.9% RH (Desiccant)	n=12	ES 20, SD 6.0	2,861 fps
34.9% RH (32% humidor pack)	n=10	ES 31, SD 10.2	2,829 fps
66.1% RH (65% humidor pack)	n=12	ES 82, SD 20.5	2,744 fps
82.3% RH (84% humidor pack)	n=12	ES 25, SD 6.8	2,669 fps

Chamber pressure:

Group Tested	# of Samples	Avg. Pressure	Peak Pressure
52.2% RH (Control)	n=12	51,608 PSI	52,545 PSI
16.9% RH (Desiccant)	n=12	54,032 PSI	55,193 PSI
34.9% RH (32% humidor pack)	n=12	52,472 PSI	53,332 PSI
66.1% RH (65% humidor pack)	n=12	50,205 PSI	51,544 PSI
82.3% RH (84% humidor pack)	n=12	49,076 PSI	50,030 PSI

Interpreting the results

Experiment 1

At its lowest humidity (14.5% RH), our 41.50 gr H4350 charge clocked in at an average 2,879 fps, peaking at 2,901 fps. Its highest humidity (83.5% RH) saw an average of 2,650 fps with a lowest velocity of 2,635 fps. The entire experiment saw an Extreme Spread of 266 fps.

Between the desiccated and 66.5% samples, a 10% change in RH resulted in a velocity change of about 25.6 fps. Above 66.5%, this effect nearly doubled and a 10% change in RH resulted in a change of about 57 fps.

There’s a similar trend with chamber pressure:

It’s important to note that the numbers observed for chamber pressure are only estimates and should not be referenced to determine the safety of this or similar loads. The PressureTrace II claims to accurately measure the relationship between samples, but correctly calculating absolute chamber pressure can only be achieved with finely calibrated industrial tools.

Experiment 2

Our second experiment largely corroborates our findings from Experiment 1:

However plotting the two velocity datasets on top of each other provides us with additional insight:

Both experiments found nearly identical velocity spreads at factory humidity levels, but as RH became more extreme the results began to diverge. Since Experiment 2 isolates moisture content as the sole variable, any difference between the two results should be attributable only to their difference in charge weight.

For instance, our 41.22 gr sample of 16.9% RH H4350 (conditioned from 41.50 gr) was 18.2 fps slower than our 41.50 gr sample at 14.5% RH. Presuming there isn’t a significant difference in the 2.4% RH spread, that means that at 14.5% RH approximately 71 fps of muzzle velocity change (from baseline) can be attributed to a change in moisture content, while the remaining 18 fps can be attributed to its effect on charge weight.

The same calculation can be applied to the other end of the graph. At 83.5% RH, approximately 121 fps of muzzle velocity change (from baseline) can be attributed to a change in moisture content, while the remaining 19 fps can be attributed to its effect on charge weight.

Closing thoughts

Some takeaways from this experiment:

Humidity has a significant impact on the performance of H4350 and likely rifle (single base) powders in general.

The “effective” charge weight of a load changes depending on powder humidity – by a few tenths of a grain, in our case with H4350.

However the largest impact by far on powder performance is driven by a change in burn rate. Only modifying a charge’s moisture content (while keeping its effective charge weight constant) has almost as much of an effect as moisture content and charge weight combined.

At the extreme ends of the scale, high RH slows down the burn rate of H4350 more significantly than low RH speeds it up.

Don’t “keep your powder dry,” keep it consistent. Store powder in a closed jug, not in a hopper, away from significant environmental changes.

If you can’t control your storage room conditions, consider using a two-way humidity pack inside the powder jug itself. Boveda 49% RH and Integra Boost 55% RH packs may be candidates for H4350.

Drying out powder doesn’t let you achieve “free velocity.” It increases the burn rate, but if you want a faster burning powder then just choose a faster burning powder. Significantly altering moisture content may create unsafe conditions or unwanted side-effects in your load.

There’s a ton of research opportunity in this area. Future researchers may be interested in testing the RH sensitivity of other popular powders, its impact on characteristics like temperature stability, or even its effect on other components like primer ignition.

There’s no way to know for sure what happened with my 6.5 Creedmoor load, however I’d been loading from the same 8lb H4350 jug since the winter. It’s very likely my powder was dryer during load development, then re-hydrated as spring and summer progressed. A 21% change in RH would easily account for the 54 fps change in velocity.

Special thanks to Brian Li and Kaitlin B. for their assistance in designing this experiment and analyzing results.

If you handload your ammunition, check out our open source software ChronoPlotter which lets you easily graph and analyze your load development sessions.

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