When IMR introduced their new line of Enduron Technology powders we knew we had to test all three. With all of the powders on the market, one can legitimately ask, why make new ones? The answer is simple, the IMR Enduron powders feature a built in copper fouling eliminator, are insensitive to temperature extremes and are composed of small sized grains for easy metering and precise loading density.

In addition, for all the tree huggers in the world, there are no ingredients considered harmful to the environment. Unless, of course, you consider blowing a three inch hole through a trophy bull elk at 200 yards harmful.

These new IMR Enduron powders were designed to meet the same temperature independent requirements as the Hodgdon Extreme family of powders, which include: H4198, H322, BENCHMARK, H4895, VARGET, H4350, H4831, H4831SC, H1000, RETUMBO and H550BMG.

Because the IMR Enduron powders cover the entire range of center fire calibers, it would not be fair to simply pick one powder for testing. Therefore, we selected five different rifle calibers to test all three powders simultaneously. This turned out to be a bigger task than we had originally envisioned, but the results were worth the effort.

IMR 4166 was the first and fastest burning of the Enduron powders. It is recommended for cartridges such as the .204 Ruger, .223 Remington, .22-250, .257 Roberts, .30-30, .308 Winchester, 8x57mm and similar numbers. We tested this powder in .223 Remington and .22-250 rifles.

IMR 4451 powder has a medium burn rate and was formulated to provide top performance in the .243 Winchester, 6mm Remington, .270 Winchester, 7mm-08, .30-06, .300 WSM, .338 Win. Magnum, .416 Rigby and many others. This powder is designed for mid-range burn rate cartridges, so we tested it in .243 Winchester and .270 Winchester rifles.

IMR 7977 powder is a magnum propellant with the slowest burn rate of the Enduron powders. It is recommended for the .25-06, 6.5mm-284, 7mm Rem. Magnum, .300 Win. Magnum, .300 Wby. Magnum and .338 Lapua. Thank goodness we do not have a .338 Lapua, as Jim's shoulder would not handle the punishment! We decided to test this powder in .300 Win. Mag. and call it good.

(Editor's Note: Any given cartridge can be used with multiple powders, which will typically change with the bullet weight selected. However, in several instances these cartridge recommendations, which are printed on the Enduron powder labels, seem strange. For example, why is IMR 4166 is specifically recommended for the .22-250 and .257 Roberts, but not the .243 and 6mm Remington, which are on the IMR 4451 recommended list? Why is IMR 4451 recommended for the 7mm-08 and .30-06, but not the .308, which is on the IMR 4166 recommended list?)

In preparation, Jim loaded sixty rounds in each caliber, selecting powder charges from the Hodgdon/IMR manual that approximated the velocities we use in our regular hunting ammunition.

All rounds were prepared with new cases, CCI primers and loaded on the same day. Each load was metered and weighed with an RCBS Chargemaster 1500 and double checked on a Frankford Arsenal DS-750 digital scale. The C.O.L. of each cartridge was set slightly under the maximum recommended in the 2015 Hodgdon reloading manual.

For those of you who are competitive shooters, the small size of the powder grains make it possible to calibrate loads to 1/100 grain. However, you will need a more precise scale than the Chargemaster for that, so we settled for 1/10 grain.

We used the same Hornady, Sierra and Nosler bullets in our tests that we load for our hunts. The specific bullet weights and types are shown in the table below.

When we headed to the range for a test, we randomly selected 20 rounds in each caliber from the inventory. We chronographed the velocities with a Caldwell Ballistic Premium chronograph using their Android phone app. Five rounds were fired to determine the average velocity for each caliber. All recorded velocities were used to calculate the average muzzle velocity and small sample size standard deviation. The remaining rounds were fired for accuracy.

The test rifles were cleaned with Otis O12-CU copper remover after each range session. While we cannot say that all copper fouling was eliminated, we can state there was less buildup than normal and what remained was easily removed with a few wet patches. That is a real plus for target shooters, who can go through several hundred rounds a day.

We fired our test rounds when the temperatures were at 43-degrees F, 75-degrees F and 97-degrees F. In order to shoot over that wide a temperature range we had to extend our testing over a two month period, from May to June 2015. For the record, Jim does not intend to ever shoot at 97-degrees F again. That session provided him a whole new meaning for the term "crispy critter."

Before we get to our results, we want everyone to know that we never doubted the claims made for the IMR Enduron powders. Our purpose was to provide reloaders with information on five popular calibers to give them an idea of what to expect, should they decide to use IMR Enduron powders.

With that being said, these new powders are everything Hodgdon claims them to be. Talk about being unaffected by temperature, we were amazed. Check out the tabular results below for yourselves.

Jim used a Caldwell Fire Control front rest and rear bag when shooting for accuracy. We found no significant difference in accuracy between shooting at 43-degrees F and shooting at 97-degrees F. The impact points on our targets were a bit higher, but it was not statistically significant.

Some groups (notably with the .223 Rem. and .270 Win.) opened up on average of 1/4 inch to 1/2 inch when comparing the targets shot at 43-degrees to those at 97-degrees. Whether that was due to the temperature change or Jim's shooting, we cannot say. Regardless of the cause, from a hunter's perspective, accuracy was essentially unchanged.

Given the velocity results, we believe these powders will perform superbly for competitive shooters. If you add the numbers, we fired 300 rounds during our tests, more shooting than we have done in any other field test in the past ten years. If there was going to be an accuracy problem with these powders as the temperature increased, it would have been revealed.

If you check our loads against the Hodgdon IMR manual, you will see that we did not load any of the calibers to the maximum. We prefer mid-velocity loads, which are easier on the barrel and not so punishing to our shoulders. Besides, the critters we are hunting are not wearing Kevlar vests.

Our tests confirm that the Enduron powders are as unaffected by temperature as any powder could be. If you are a reloader and you shoot in a wide range of ambient temperatures, you owe it to yourself to try these IMR Enduron powders.