From what I had read on the internet, which is a repeat of what is said in gun magazines, powder has an “indefinite” shelf life. Remember reading statements to the effect that powder lost energy as it got old, making it essentially benign.
Then I ran into an Insensitive Munitions expert. This IM expert explained that powder deteriorates from the day it leaves the factory.
Nitrocellulose decomposes through the reduction-oxidation process. Called Redox. The expert said “The molecular stability of the functional groups on the organic chain determine the life time of the nitrocellulose molecule.” All ionic compounds, water is the main offender because it is always in air, react with those bonds and accelerates the deterioration of the powder.
The bottom line is that nitrocellulose is a high energy molecule that wants to become a low energy molecule.
Heat accelerates the deterioration/decomposition of powder and the rate is directly proportional to the Arrhenius equation. If you read in the Insensitive munitions literature, you will see that they use high temperature to accelerate aging of smokeless propellants.
Double based powders have a reduced lifetime compared with single base. Double based powders have nitroglycerin (NG) in the grain. Nitroglycerine remains a liquid and it migrates within the grain to react with the NO bonds on the nitrocellulose, increasing the rate of reduction-oxidation reaction. All ionic compounds react with those bonds and accelerate the deterioration of the powder. Rust is bad as ferric oxide is ionic. Water is ionic and it is ever present in the air.
Because water reacts in a negative way with smokeless propellants, quality ammunition is manufactured in humidity controlled environments. Between 40% and 20% humidity. They don't go lower due to electro static discharge concerns.
The best storage condition for powders is arctic. Cold and dry.
Due to the migration of NG within double based powders, the surface of the grain will become rich in NG even though the total energy content of the propellant has decreased. This will cause changes in the burn rate, and can cause pressures to spike. The surface of nitrocellulose powders also change as the powder deteriorates, and it changes unevenly. This creates conditions for erratic burn rates. Burn rate instability is undesirable and can cause explosive conditions in firearms. In retrospect, this explains the “funny” retorts I experienced and the sticking cases. It is an extremely rare occurrence, but old ammunition has caused rifle Kabooms. When I discussed this with a machine gunner buddy, he said that explained the two top cover explosions he had with old Yugoslavian 8 MM ammo.
Section from the US Army Propellant Management Guide:
Stabilizers are chemical ingredients added to propellant at time of manufacture to
decrease the rate of propellant degradation and reduce the probability of auto ignition during its expected useful life.
As nitrocellulose-based propellants decompose, they release nitrogen oxides. If the nitrogen oxides are left free to react in the propellant, they can react with the nitrate ester, causing further decomposition and additional release of nitrogen oxides. The reaction between the nitrate ester and the nitrogen oxides is exothermic (i.e., the reaction produces heat). Heat increases the rate of propellant decomposition. More importantly, the exothermic nature of the reaction creates a problem if sufficient heat is generated to initiate combustion. Chemical additives, referred to as stabilizers, are added to propellant formulations to react with free nitrogen oxides to prevent their attack on the nitrate esters in the propellant. The stabilizers are scavengers that act rather like sponges, and once they become “saturated” they are no longer able to remove nitrogen oxides from the propellant. Self-heating of the propellant can occur unabated at the “saturation” point without the ameliorating effect of the stabilizer. Once begun, the self-heating may become sufficient to cause auto ignition.
The Armed Forces have stockpile surveillance programs but each Service does theirs a little differently. If you want to see all the different tests the military uses to determine propellant characteristics, look at Mils Std 286 Propellants, Solid: Sampling, Examination and Testing to be found at
https://assist.daps.dla.mil/quicksearch/.
If you look, you will find aging tests. One common test is for powder to be kept at 65 C until it fumes. It if fumes within 30 days it is checked for stabilizer or scrapped.
The Navy expert told me a few ways the Navy samples its powders and propellants. If the powder is outgassing nitric gas (as determined by change of color of methly violet paper in contact with the powder (Methly Violet test, or Talliani test)), the stuff is tested to see how much stabilizer is left. If the amount is less than or equal to 20%, the lot is scrapped.
Scrapping powders and propellants with this percentage of stabilizer appears to be consistent across all services.
Pages 5-11 of the 2003 Army Logistics Propellant Management Guide provide the protocols for testing and subsequent actions for their Stockpile Propellant Program. Basically, all propellant lots are tracked. The trigger for investigation is: "When Master Sample Stability Failure Occurs"
The Navy expert provided 'rules of thumb' concerning when to expect problems with double based and single based propellants. The rules of thumb are: Double based powders and ammunition are scrapped at 20 years, single based 45 years. In his words “These 'rules of thumb' are particularly useful when the protocol fails. The protocol can easily fail when workmanship or good housekeeping measures are not followed during manufacture of propellant and/or rocket motor or during storage of the weapon system components, respectively.”
The expert suggested that it is likely that surplus military powders are not on the market anymore due to liability issues. The stuff was scrapped because the military decided it was not safe to keep around anymore.
For the home reloader, if the powder has turned red, or smells like acid, it is way beyond its safe limits.
I am of the opinion that the reason this is not discussed in the popular gun press is because if the shooting community knew that powders had a shelf life, it might effect sales. As we all know, gunwriters are shills for the industry and for decades the shills have been reassuring us that as powder gets old, it becomes benign. I cannot see a reason why industry wants you, the shooter, to be picky about old powders and old ammunition. You might not buy, you might have reservations about buying. It is all about profits you know.
More to read if you wish:
www.dtic.mil/dticasd/sbir/sbir031/n154.doc
This paper discusses most of what I have written, but it has a confusing section where it states that “Suddenly, propellant that has spent its entire life in a configuration that was considered inherently safe from the risk of auto ignition is now bulk packaged and stored in a concentrated mass that may be sufficient to allow auto ignition to occur.” After discussions with the Navy expert I found that the insensitive munitions community has its own myths and legends. There are groups within the IM community who promote the “5 inch” rule. The theory is that for munitions 5 inch and smaller, the thermal mass of the case is sufficient to wick away heat and prevent auto ignition. The Navy expert considers this theory to be bogus and created by self serving individuals who get cash awards when they “extend the shelf life” of propellants. Never doubt the power of greed.
http://www.almc.army.mil/alog/issues/JulAug08/propellant_stab_eq.html
An example of powder that went bad in the can:
http://thefiringline.com/forums/showthread.php?p=4702804#post4702804
