March 10, 2021
By Dave Emary
Have you ever opened a can of powder and gotten a puff of foul-smelling brown smoke? Have you shot ammunition in the winter without issue, only to experience stiff bolt lift and cratered or pierced primers in the summer heat? Have you sighted in during a warm day and later had a round impact off your zero on a cold one? Or, have you wondered why an ammunition or propellant may perform differently over time?
These issues are directly attributable to changes in chemical reaction rates caused by temperature and the propellant’s exposure to high heat. The chemical stability and life of propellant is decreased with relatively short exposure to high temperatures.
Short-Term Temperature Effects on Propellant
The burning of propellant is a chemical reaction process. Like most chemical reactions, the initial rate of the reaction is affected by the initial temperature of the process. The change in the reaction rate as a function of temperature is also strongly dependent on the composition of the materials in the reaction. Generally speaking, the propellant in ammunition will lose pressure and velocity as it gets colder, and it will gain pressure and velocity as it gets warmer. There are some exceptions that I will discuss later.
The initial burn rate of propellant is affected by the temperature of the propellant. If a propellant is at 70 degrees Fahrenheit (F) and then heated to 120 degrees F, the rate that it initially starts burning will be faster at 120 degrees than it was at 70. If the propellant is loaded in ammunition, the increase in burn rate will cause higher pressure and velocity at 120 degrees than at 70 degrees. At 0 degrees, the initial burn rate will be slower and the propellant will produce lower pressure and velocity.
A propellant’s ingredients factors into how much its performance changes with temperature. Consider a double-base propellant, or a propellant that contains nitroglycerine. The nitroglycerine is added to increase the energy content and performance of the propellant. The higher the energy, the more velocity the propellant is capable of. The drawback is that the reaction rate of nitroglycerine is sensitive to temperature. Older design double-base propellants have significantly more change with temperature than a single-base propellant lacking nitroglycerine.
Ball powders of the past are known for large variations in performance from low to high-temperature conditions. Some cartridges can have dangerous performance with Ball powder in hot temperatures such as WC 760 in a .22-250 Remington, .220 Swift or .243 Winchester. It is common with older Ball powders to get upwards of 150 to 200 feet-per-second (fps) changes on either side of ambient temperatures. Stick, or extruded propellant generally offers better temperature sensitivity than Ball powder.
Recent advancements in propellant chemistry and manufacturing techniques produced improvements. Extruded propellants are now available in both single and double base featuring nearly stable performance across a range of temperatures. Examples of these are Hodgdon's Extreme line, recent propellants from Alliant Powder such as Reloder 16, 23, 25 and 33 and newer IMR propellants such as 4166, 4451, 4955 and 7977. This is due to the chemical composition and primarily because of new manufacturing techniques that allow the perforation through the extruded propellant to be covered by the thin layer of burning inhibitor, called a “deterrent,” applied to the surface of the propellant grains. Compare the appearance of the old IMR 4895 and the newer Alliant RL-16. The grains of IMR 4895 shows the single visible perforation running all the way through the center. The RL-16 only possesses a hint of an indentation where the perforation is on some of the grains. The perforation is covered by the surface-deterrent coating, but the perforation is there. The covering of the perforation reduces the initial burning surface available until the thin layer of deterrent is burned through. This slows the initial gas production rate of the propellant and offsets the effects of increased burning rate at elevated temperatures.
Recent Ball powder designs, such as the Hornady Superformance propellants, use different types of surface coatings that act as both a flash suppressant and an initial burn rate inhibitor at elevated temperatures. The degree to which these propellants are flattened is carefully controlled to slightly stress the grains. This improves the cold temperature performance of the Ball powders. The stressed grains at cold temperatures tend to fracture somewhat, which increases the burning surface area slightly, increasing the gas production rate and reducing the pressure and velocity drop at cold temperatures. There is a fine line though. If the propellant grains are flattened too much, the pressure at cold temperatures can go up substantially. The Superformance propellants reduce the temperature-related performance changes of Ball powder by a half to two-thirds of what older designs produced. As good as they are, for a Ball powder, the Superformance propellants cannot match the temperature stability of the newer extruded propellants such as the Hodgdon Extreme line. The trade-off is that Ball powders safely produce higher performance than extruded propellants.
In shotgun powders, the chemistry often results in a brittle grain of propellant at cold temperatures that tends to fracture, which can actually slightly increase pressure at cold temperatures to prevent a change in pattern or point of impact. Ball powder shotshell propellants are excessively flattened in order to slightly crack and stress the propellant grains and achieve the same cold temperature effect as described above. Table 1 shows the velocity changes of some popular propellants in the .300 Winchester Magnum at temperatures of 0, 70 and 100 degrees F.
If you are participating in hunting or shooting that could take place in a range of temperatures, consider using propellants that have shown stable performance across the temperature range. If you are a long-range hunter or competition shooter, this is a very important consideration.
Propellants have an expectation of having a shelf life of many decades. The primary components of propellant will not last this long without deteriorating or without chemical stabilizers added to them. These stabilizers are slowly used up over time. The rate of the consumption of these stabilizers is dependent on the temperature of the propellant. The higher the temperature, the more rapidly the stabilizers are used. Once the stabilizers are consumed, the propellant can get to the point where it will ignite spontaneously, also known as a “cook-off.” Table 2 shows the decomposition, or cook-off time, for double-base propellant as a function of temperature.
There have been instances of propellant being stored in high-temperature environments for long periods of time (months) with negative consequences. A good way of assessing your propellant is to smell it. If it has a sweet smell like acetone, ethanol or other solvents, then it's fine. If your open propellant container smells nasty (like smelling salts), or the contents are turning a reddish-brown color or producing a reddish-brown dust, get rid of it!
Do not store your propellant or ammunition in your car or garage during the summer. The explosive mix in your primers suffers from the same effects as propellants. Always keep propellant, primers and ammunition in a cool, dry place for long-term storage as much as possible. Several months of exposure to high temperatures (above 100 degrees F) can substantially shorten the life and the expected performance of propellant. Cold temperatures have little effect on propellant. In fact, the best possible place to store your propellant would be in a freezer.
Another effect of weeks-long propellant exposure to high temperatures is called “deterrent migration.” Under elevated temperatures, above 100 degrees F, the deterrent on or in the propellant can spread further into the grains of propellant. This movement of the deterrent spreads it out within the propellant grain and decreases the amount of deterrent near the surface. The movement of the deterrent causes the burn rate of the propellant to change and get faster, potentially causing higher pressures. Older Ball powder designs are particularly susceptible. Ammunition left in a hot car may not destabilize it, but it can cause high and unsafe pressures.
Take Care of Propellant and Ammunition
After reading this article, don’t be paranoid about how you treat or take care of ammunition and propellant. Just know the limitations. Don’t leave ammunition in your car for long periods. Do not store your reloading powders, primers or ammunition in a garage unless it is climate controlled. Propellant will hold up well for short periods of time with exposure to temperatures up to about 100 degrees F. Respect the propellant and you won’t have any problems.
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