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Primers and Pressure
The effect of ambient temperature on cartridge performance is often considered by shooters who venture out into extreme environments. While powder performance at high ambient temperatures is an often discussed topic, primer performance at these same extremes is less commonly examined.
To test the effect of temperature on primers, we chose three magnum rifle primers: the CCI 250 Magnum Rifle Primer, the Winchester Large Rifle Magnum and the Federal 215M Large Rifle Match primers and loaded them into .338 Lapua Magnums loaded under laboratory controls.
The .338 Lapua Magnums were loaded with 300 grain Sierra Match Kings using 95 grains of a non-canister propellant commonly used by the larger OEMs. Testing consisted of three sets of twenty cartridges loaded identically except for their primers. Half of each group was kept at 70° F while the other ten were placed in a laboratory oven set to 140° F. for three hours. Only the primers differed between tested groups.
The control group averages were very close between the three tested primers. The maximum average spread was ten feet-per-second between the fastest and slowest loads. By pressure the difference was 1060 psi between the highest and lowest loads, a difference of 1.8%. The primer that produced the highest average pressure and velocity in this loading was Winchester’s Large Rifle Magnum at 2707 fps and 59,720 psi and the lowest was the CCI 250 which clocked 2697 fps and 58,660 psi.
Adding heat also added pressure and velocity to all of the rounds tested at 140° F. The most consistent of the three tested groups was powered by Federal 215M primer which showed an increase of 12.7% in pressure and a velocity increase of 2.7%. Even though it produced the most stable results, pressures were still quite high registering 66,190 psi.
The load using the CI 250 primer showed an increase of 9,570 psi (a 16.3% increase) over the 70° control group, averaging 68,230 psi. There was also a 3.6% increase in velocity from 2,697 to 2,793 fps.
The Winchester WLRM primer produced the largest increases, with velocity moving up to 2,820 fps and pressure increasing to an average 70,510 psi, an 18% increase in pressure.
What Does it Mean?
Pressure increases associated with higher ambient temperatures are common with most propellants, especially at ranges above 120° F. Cartridges will begin to cook off as their internal temperatures reach about 300° F. In our test, the main pressure increase comes from the higher temperature’s effect on powder combustion, not from differences in primer ignition characteristics. However, since the only difference between the groups was created by primer selection, it is interesting to note the changes in pressure.
Handloaders tend to overlook differences between primer ignition characteristics and how they may effect pressures on otherwise well-tested loads. It is important to remember that handloads are not static items; they can be affected by changes in ambient temperature, no matter what the advertising departments may hope to have you believe. These pressure changes associated with primer ignition characteristics highlight a rule that all handloaders should know and follow: Whenever components are changed from a known handload, it is important to return to the starting load and work it back up watching for obvious pressure signs. We all know it. Looking at the differences between primers should remind us why.