January 15, 2022. Most shooters are aware that temperature changes can affect the performance of powders. You should keep loaded ammo out in the sun and keep rounds in a hot chamber until you are ready to fire. There are other factors that must be considered, such as humidity. This article will explain why and how humidity can impact powder performance and burn rates. Norma has shown that even the smallest environmental variations in humidity can have an impact on how powders burn. This is at least for the long-term. Sven-Eric Johansson is the head of ballistics at Nexplo/Bofors and provides a detailed discussion on water vapor absorption by powder in the Norma Reloading Manual. Johansson shows that powders with different water contents will burn at different rates.
Johansson explained that most powders contain 0.5-1% water by weight when they are manufactured. To maintain the 0.5-1% moisture content, the relative humidity is “equilibrated” at between 40-50% and 50% during manufacturing. Johansson points out that powder exposed for long periods to moist air will absorb water and cause it to burn slower. However, powder that has been stored in dry conditions for a long time will burn faster because of its lower moisture content. In addition, Johansson found that single-base powders are MORE sensitive to relative humidity than are double-base powders (which contain nitroglycerine).Tests Show Burn Rates Vary with Water Content
Fred Barker’s review of the Norma Manual notes that Johansson provides twelve plots of the velocities, pressures and velocity of popular cartridges using Norma powders. These range from #200 to MRP. He also provides results for loaded.30-06 cartridges and.38 Special cartridges that were kept at relative humidity levels of 20% and 86% for 663 to 683 day. Johansson advises that powders be kept tightly sealed in their factory containers and to avoid exposure to dry or humid atmospheres. Barker confirms Johansson’s findings that storage conditions can alter the burn rate. Barker says: “I have about 10 lbs of WWII 4831 powder that I have stored in dry Colorado air (about 20% RH), for over 60 years. It now burns approximately like IMR 3031. What does this tell us? First, all powders have a small but important amount of water content. The powder’s water content will change depending on how it is stored. Third, the water content of your powder can make a big difference in how it burns, especially for single-base powders. These factors are important to remember for reloaders. Johannson recommends that powders be kept tightly sealed when not dispensing charges. Powders should also be kept out of very dry or humid environments to preserve their “as manufactured” burn rate. MidwayUSA sells the Norma Reloading Manual for $24.99. Robert writes: “I had an eight-lb. I kept the jug of H4831SC in my detached garage because it can get humid there. This was 43.5-44.0 grams and it was very accurate with the 115 Bergers from my 6mm Super X. After a few weeks, I took the jug and kept it in my detached garage. I went to a match with 43.5 gr and it shot terrible. I was unable to figure out why it was happening until I checked the chronograph again. I found that the load was moving a lot faster than normal and was out of the “sweet spot”. (42.5 – 43.0 grams was the maximum I could load and keep it accurate in humid air). After a few weeks, I returned to loading 43.5-44.0 gr. It shoots great again. This has happened with other powders. Similar Posts:Tags: Burn Rates, H4831, Humidity, Johansson, Moisture Content, Norma, Powder Storage, Propellant. If you have two jugs with the same powder, one in your home and one elsewhere, don’t expect them to chrono the exact same amount of powder with the same charge weights until they are both stored in the same place long enough to equalize. Similar Posts:Tags: Burn Rates, H4831, Humidity, Johansson, Moisture Content, Norma, Powder Burn Rate, Powder Storage, Propellant
