5.6  Muzzle Velocity Dependence on Cartridge Temperature

5.6  Muzzle Velocity Dependence on Cartridge Temperature

  (CAUTION: The tests described in this section were conducted under carefully controlled laboratory conditions. Sierra recommends strongly that readers should not attempt to conduct such tests, because a safety hazard can exist.)

The muzzle velocity developed by a cartridge/load combination depends on the temperature of the cartridge (and its primer and power) at the instant of firing. This dependence can be important. Although it is really an interior ballistics effect, the variations in muzzle velocity cause changes in exterior ballistics performance which are important to understand.

The general trend in muzzle velocity versus temperature is that the higher the temperature of the cartridge and its components, the higher the muzzle velocity. Of course, the rise in muzzle velocity is caused by a rise in chamber pressure, and this is a possible safety hazard for loads which are near the upper extreme of safe chamber pressure at normal shooting temperatures.

Sierra conducted a series of tests with three popular cartridges for rifles and handguns to investigate the dependence of muzzle velocity on cartridge temperature. The three cartridges chosen were the .30-’06 Springfield, the .243 Winchester, and the .44 Magnum. The test cartridges, conditions, and the results are shown in Tables 5.6-1 Tables 5.6-2 Tables 5.6-3 and Tables 5.6-4 . The .44 Magnum cartridge was tested with two bullets from Sierra’s line, the 180 gr and 240 gr JHC. Two powder types, a ball powder and an extruded powder, were tested for each cartridge.

The test procedure was as follows. The bullet chosen for each cartridge/load combination was a popular hunting bullet in each case. The choices were the 165 gr SBT for the .30-’06, the 100 gr SBT for the .243 Winchester, and the 180 and 240 gr JHC bullets for the .44 Magnum. For each rifle cartridge 30 rounds were loaded identically for each powder type (total of 60 rounds per cartridge type). All loading was performed in the laboratory at the ambient temperature (72 degrees F). Then, the 30 rounds for each powder type were separated into 3 groups of 10 rounds each. One group of 10 rounds was maintained at laboratory ambient temperature as a control group. One group of 10 rounds was cold-soaked in a freezer at 20 degrees F for several hours, while the remaining group of 10 rounds was soaked in a warm oven for several hours at a temperature near 135 degrees F. These temperatures were chosen to simulate conditions that might be encountered when cartridges are carried in an exposed backpack in the northern, mountainous regions of North America, or stored in the trunk of a car in the hot southern deserts, or left in the open sunlight on the bench at the local shooting range.

Each test was conducted by first firing the 10 round of the room temperature control group as quickly as possible, recording the muzzle velocity for each round. Then, the 10 rounds in the freezer were removed and fired as quickly as possible, recording the muzzle velocities. Finally, the 10 rounds in the oven were removed and fired as quickly as possible, recording the muzzle velocities. It is not likely that the temperatures of the cold and hot rounds changed by more than one or two degrees during the firing sequence, which took well under two minutes for each group of 10 rounds.

The tests of the .44 Magnum handgun cartridges were conducted in exactly the same way, except that the size of each group of rounds was 5 instead of 10.

For each cartridge/load combination tested on a moderate powder charge was chosen, so that the chamber pressures produced by the heated and cooled cartridges would not vary beyond the safe limits for chamber pressure. It must be clearly understood that any conclusions drawn from the test results shown in Tables 5.6-1 Tables 5.6-2 Tables 5.6-3 and Tables 5.6-4 apply for the moderate loads listed in the tables. Perhaps the most important conclusion from these tests is that much more testing is needed to precisely quantify the effects of cartridge temperature on muzzle velocity, especially at near maximum loads.

However, the following conclusions can be drawn from the data in Tables 5.6-1 Tables 5.6-2 ,Tables 5.6-3 and Tables 5.6-4 :

(1) A c thidge has a muzzle velocity sensitivity to cartridge temperature of about 1 fps per degree Fahrenheit. These are only approximate, and they apply only for the moderate loads used.

(2) The muzzle velocity sensitivity to cartridge temperature for the two ball powders used in these tests does not appear to be much different than the sensitivity for the extruded powders used.

The consequent effects on ballistics performance of the cartridges tested are measurable, but they are not especially large. For example, here in Southern California hunters sight in their rifles at outdoor ranges where temperatures are typically in the mid-80’s to mid-90’s. Many of these hunters go after deer in Northern California where early morning temperatures during hunting season can be in the 30’s and 40’s. This amounts to a temperature difference of about 50 degrees F between the shooting range and the hunting territory. If a hunter were using a .30-’06 cartridge with one of the loads shown in Table 5.6-1 , the change in temperature would cause the muzzle velocity of his rifle to lose about 100 fps, from near 2700 fps at the shooting range to about 2600 fps at the hunting location. If we look at the Ballistics Tables for the .308 diameter, 165 gr SBT bullet, we see that drop increases by about 0.8 inch at 200 yards, 2 inches at 300 yards, and not quite 4 inches at 400 yards. In other words, the hunter can expect his rifle to shoot lower than it does at his shooting range by those amounts at those ranges.

If the hunter were to use a .44 Magnum handgun against, for example, feral pigs in Northern California, the same change in temperature would cause the muzzle velocity to be decreased by about 50 fps for any one of the loads in Tables 5.6-3 or 5.6-4 . If we look at the Ballistics Tables for the .4295 dia., 240 gr JHC bullet at 1400 fps, we see that drop increases by approximately 0.8 inch at 100 yards, 1.25 inches at 125 yards, 1.8 inches at 150 yards, and 4.9 inches at 250 yards. So the handgun hunter will find that his pistol shoots lower than his reference trajectory by those amounts at those ranges because of the change in temperature.

For typical hunting scenarios like those in the examples above, changes of 50 degrees F or so between the shooting range where the rifle or handgun is sighted in an the hunting location should not be the sole cause of misses, unless the hunter fires at game which is a very long distance away. It is not impossible, however, to have a change in temperature of 100 degrees F between the shooting range and the hunting territory here in North America. If you live in any of the southern, southeastern, or southwestern states, it would not be unusual to sight your rifle or handgun at a shooting range where the temperature is around 100 degrees F. Then if you hunt in Alaska, Canada, or the northern continental U.S., you can encounter weather conditions with temperatures near 0 degrees F. The decrease in muzzle velocity caused by the 100 degree F decrease in temperature would approach 200 fps for rifles and 100 fps for handguns, with the loads listed inTables 5.6-1 Tables 5.6-2 Tables 5.6-3 and Tables 5.6-4 . The changes in drop then become considerably more important.

It should also be mentioned that a change in temperature can be caused by a change in altitude, as well as a change in weather, between the shooting range and the hunting location. At higher altitudes any gun will tend to shoot flatter (less drop). But at lower temperatures (which go with higher altitudes) the cartridge temperature tends to make the bullet trajectory have more drop. Thus, for a temperature change that occurs because of an altitude change, these effects offset each other partially.

In closing this section we repeat our earlier admonition. All the data in this section are for moderate loads. If you are using a handload which is near the maximum level recommended in the Loading Data section of this Manual, be aware that high cartridge temperatures can elevate chamber pressures. Use care and caution in developing your loads. When you are at the shooting range, do not allow unfired rounds to lie in the open sunlight for extended periods of time.