Temperature Sensitivity of AR2225 Rifle Powder.

Otherwise known as "Retumbo"

It is well known that changes in temperature have an effect on projectile velocity and chamber pressure. But how much effect? After some interesting long range shooting results, in sub zero temperatures, I thought that I would conduct some simple tests to see if I could find a trend to what was going on with the particular ADI powder that I was using at the time. The powder I was using was AR2225. ie "Retumbo" Now this powder is claimed to be temperature stable by ADI limited. So before I conducted these tests, I contacted the ADI Technical Centre, who gave the following reply as shown below.

"ADI propellants have a built-in technology which provides superb ballistic thermal insensitivity. The change in velocity over the temperature range of -0°C to 50°C is generally less than 30 fps. Unfortunately, we have no data measured in the particular firearms you mention: we perform our testing using test barrels in a controlled, laboratory environment.  The components used will also effect the ballistic thermal insensitivity. Extreme caution should be taken and loads should be worked up accordingly.  Refer to our website at http://www.adi-powders.com.au   or our 5th edition handloaders’ guide for more information, warnings and reloading safety".

Now this reply sounds quite reasonable but I was after more detailed results with my particular firearm. So I decided to run a few simple tests to try and narrow down the magnitude of any possible velocity range that I might encounter. Temperatures in Australia vary quite a bit from the freezing cold of the Great Alpine Ranges in winter to the blistering heat of the central plains. I carefully loaded up a bunch of cartridges with a load that has proven to be quite safe in my rifle with moderate velocity and pressure. All the charge weights were within 0.1 grains of accuracy.  Then to simulate a wide range of temperatures I placed five of the cartridges into the freezer to cool off for a few hours.  Then I prepared a hot water bottle and filled it with boiling water. Another 10 of the cartridges were placed against the hot water bottle and wrapt up in a thick towel to get nice and toasty. I then pulled out the ice cold cartridges out of the freezer and wrapt them up in ice and a separate towel for insulation. With the ammo all sorted, I then quickly headed down to my private range to see how these cartridges loaded with AR2225 powder would perform over the chronograph. The results were interesting! I started off by first shooting the cold cartridges in a cold barrel and recorded the velocity of each shot, letting the barrel cool for a few minutes between each shot. These cartridges were still well below zero when fired. I then fired five cartridges at the atmospheric temperature of 24 deg C and recorded the velocity. Time was given to cool the barrel between each shot. Then it was time to shoot off the hot cartridges. Now these cartridges came out of the towel way too hot to hold and I estimated the temperature of these cartridges at about 70 deg C. As I had 10 of these cartridges, I let a few of these cartridges cool down slightly until they were cool enough to hold but still quite warm. As these rounds did not produce any sign of excessive pressures, I then shot five of the hottest of the sample cartridges.  The results were very interesting. The hot cartridges had a very low ES and were all very consistent. The "room temperature" cartridges average velocity was 12 fps less than the "hot cartridges". Three of the "cold cartridges" were only about 15 fps slower than the "hot cartridges". However I had two of the five cold cartridges that recorded velocities that were way slower than the other three, one by 80 fps and one by 90 fps! This seems to agree with last years test where we found the odd shot that landed way low at long range, in sub zero temperatures. Maybe this powder has poor ignition in below zero temperatures? Maybe the result is due to the primer? Whatever the reason, it is something that needs to be avoided. From this test I would say that this powder seems to be very consistent with its velocity versus temperature change for mild and hot climates. The powder actually seems to work very well when hot. Conclusion :-  The exact details of this test were kept a bit vague on purpose due to the potential danger that this test could pose if anyone chose to copy it with an unstable powder. The main outcome of this test was not to come up with any exact results, but rather to look for any trends and abnormal occurrences that may result from such a wide variation in temperatures. There did not appear to be much variation in velocity or pressure with this powder under normal operating temperatures and the claim by ADI that this powder is temperature stable seems to be quite reasonable even given the small sample size. However the abnormal velocity variation for two of the cold cartridges showed that it would be an advantage to keep your ammo warm in your pocket when shooting in sub zero temperatures. Do not try to repeat this test. It could result in unsafe high pressures. I plan to repeat this test with other powders to see if any useful information can be gained to aid the long range shooter. So come back from time to time to check up on my results. AR2217 ie H1000 will be next.

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