Primer Effectiveness Study

Completed 5/7/04

5/7/04

Introduction

While working up loads for my 6mm-250AI, I noticed that I was getting erratic groups and large standard deviations of velocity with the slower powders, particularly H4831SC. I had been using CCI BR2 benchrest large rifle primers for my 220 Swift and BR4 benchrest small rifle primers for my 223AI with good success. However, I decided to see if changing the primer would make any quantitative difference.

According to some sources available on the Internet, the CCI BR2 primers, while very consistent, are not very hot. My theory was that the slower burning powders were not getting well lit before the chamber volume started increasing as the bullet moved down the barrel, causing erratic burns. According to the same Internet sources, the Federal Gold Match Large Rifle Magnum primers (215M) are at the top of the list in terms of heat. Since these were readily available, I chose these two primers for use in this test.

The Test

For this test, I used previously necked up and fireformed Norma 22-250 brass, body sized with a custom Redding body die, and then neck sized with a custom Lee collet die. The bullet used was a 107 grain Sierra MatchKing, seated at 2.700” OAL with a 6mm-250AI Redding Competition seating die. The Redding seating die was reamed out to 6mm-250AI using the same reamer that chambered the rifle. The brass, including the primer pockets, was cleaned before the sizing process after decapping with a universal decapping die. The test rounds were primed and loaded at the range, as is my usual procedure for working up loads. Each load was thrown from a RCBS powder measure previously calibrated with a RCBS 5-0-5 beam scale. The RCBS measure is equipped with the optional micrometer charge stem.

I instrumented the barrel of the 6-250AI with a strain gage and a Southwest Products PressureTrace data acquisition module. I used my Oehler 35P to measure the muzzle velocities of every shot, and entered the resulting velocity data into the PressureTrace software.

I chose three different powders for this test across a range of burn rates, with IMR4895 as the fastest, H4350 as the medium, and H4831SC as the slowest. All had shown decent results with the BR2 primers, except for erratic velocities and groups with H4350 and even more so with H4831SC. All three loads were previously tuned for best groups, and yielded about the same muzzle velocity. Five shots for each powder and primer combination were tested.

Results

The results were very interesting. The 215M primers definitely make a significant difference with the longer burning powders, particularly the H4831SC. Note that the PressureTrace data often shows a higher pressure and slightly delayed trace for the first shot of the group. While this is considered normal, it is unknown why this occurs if it is an actual pressure deviation, or if it is a result of some condition in the electronics. However, when this does occur, there is no indication of an abnormal pressure or burn from either the chronograph or the bullet point of impact. I chose to ignore this anomaly when analyzing these data.

IMR4895 Test Case

Figure 1, Figure 2, and Table 1 shows plots of chamber pressure from PressureTrace and the associated statistics for the IMR4895 test loads. In this case one can see that the pressure curves for the BR2 primers are not as uniform as with the 215M primers. In particular, the peak pressures show a significant variation from shot to shot. Of particular interest is the anomalous secondary pressure event shown on traces 4 and 5 of the BR2 test case. This is apparently associated with incompletely lit powder being blown down the barrel, and then igniting as the pressure drops near the minimum as the bullet just leaves the muzzle. While the mean velocity and mean peak pressures did not change significantly, the velocity and pressure deviations did significantly reduce when the 215M primers were used. The observed group sizes were essentially equal.

Figure 1 - 35.0 gr. IMR 4895 with BR2 Primers

Figure 2 - 35.0 gr. IMR 4895 with 215M Primers

Table 1 – IMR4895 Velocity and Pressure Statistics

H4350 Test Case

Figure 3, Figure 4, and Table 2 shows the data for the H4350 test loads. Other than the previously mentioned “first trace” anomaly, it is again clear that the 215M primers resulted in a more regular pressure curve, and slightly higher and tighter velocities. The anomalous secondary pressure events are missing, as expected with this longer burn rate powder. The observed group sizes showed that the BR2 case was slightly better than with the 215M, due to the higher than desired muzzle velocity of the 215M loads. Please note that subsequent to this test, when this load was re-optimized using the 215M, I found that the optimum charge was actually 39.5 gr H4350. The hotter primer burned the powder more efficiently, and the one half grain reduction was needed to place the barrel time at the optimum of 1.257 mS.

Figure 3 - 40.0 gr. H4350 with BR2 Primers

Figure 4 - 40.0 gr. H4350 with 215M Primers

Table 2 –H4350 Velocity and Pressure Statistics

H4831SC Test Case

Figure 5, Figure 6, and Table 3 shows the data for the H4831SC test loads. As initially suspected, the use of the hotter primer clearly shows in the higher peak pressures, higher velocities, and more even pressure curves. Again, the observed group sizes for the 215M loads were slightly larger than with the BR2, again due to a higher than desired muzzle velocity. This load was not re-optimized, since the charge was fairly compressed, and QuickLoad shows that this powder did not yield as high a ballistic efficiency as the H4350.

Figure 5 - 43.5 gr. H4831SC with BR2 Primers

Figure 6 - 43.5 gr. H4831SC with 215M Primers

Table 3 – H4831SC Velocity and Pressure Statistics

Figure 7 through Figure 10 show the velocity and pressure statistics as a function of the primer type for each test case. The trends discussed in the individual test summaries can be clearly seen. In particular, the 215M primers consistently result in higher velocities and lower velocity deviations than the BR2 primers.

Figure 7 – Mean Velocity vs. Primer Type

Figure 8 – Standard Deviation of Velocity vs. Primer Type

Figure 9 – Mean Pressure vs. Primer Type

Figure 10 – Standard Deviation of Pressure vs. Primer Type

Summary

I have concluded that the use of the Federal Gold Match Large Rifle Magnum primers is an excellent choice for the 6mm-250 AI cartridges when using longer burning powders.